New [ReadOnly]Memory<T>.Cast<TFrom, TTo> extensions (#3520)

## PR Type
What kind of change does this PR introduce?
<!-- Please uncomment one or more that apply to this PR. -->

 - Feature

## What is the current behavior?
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->
Right now there is no (easy) way to cast a `Memory<TFrom>` instance to a `Memory<TTo>` instance. There are APIs to to do that for `Span<T>` instances, but not for `Memory<T>`. The reason for that is that with a `Span<T>` it's just a matter of retrieving the wrapped reference, reinterpreting it and then adjusting the size, then creating a new `Span<T>` instance. But a `Memory<T>` instance is completely different: it wraps an object which could be either a `T[]` array, a `MemoryManager<T>` instance, etc. The result is that currently there are no APIs in the BCL nor in the toolkit to just "cast" a `Memory<T>`.

This feature has been requested by a number of developers, including in a well known library such as `ImageSharp`:

> Yes, that's exactly what I would need. But I'm wondering how would you implement it.
> It's certainly non trivial to cast a `Memory<byte>` to a `Memory<TPixel>` and if there's an API for that I would gladly want to know...
> So I pressume `ImageSharp` would need to do some work under the hood.

(_`ImageSharp` issue, [here](SixLabors/ImageSharp#1097 (comment))
To solve that, I created a very simplified version of the code included in this PR, into a PR [here](SixLabors/ImageSharp#1314).

Having this available right out of the box in the `HighPerformance` package would be helpful in a number of similar situations, especially with `Memory<T>` APIs becoming more and more common across libraries now (as they've been out for a while).

## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->
This PR includes 4 new extensions for the `Memory<T>` and `ReadOnlyMemory<T>` types that enable the following:

```csharp
// Cast between two Memory<T> instances...
Memory<byte> memoryOfBytes = new byte[128].AsMemory();
Memory<float> memoryOfFloats = memoryOfBytes.Cast<byte, float>();

// ...any number of times is needed
Memory<int> memoryOfInts = memoryOfFloats.Cast<float, int>();
Memory<byte> backToBytesMemory = memoryOfInts.Cast<int, byte>();

// Or just convert into bytes directly
Memory<int> sourceAsInts = new int[128].AsMemory();
Memory<byte> sourceAsBytes = sourceAsInts.AsBytes();

// Want to get a stream from a string? Why not! 😄
using (Stream stream = "Hello world".AsMemory().AsBytes().AsStream())
{
    // Use the stream here, which reads *directly* from the string data!
}
```

Here is the full list of the new APIs introduced in this PR:

```csharp
namespace Microsoft.Toolkit.HighPerformance.Extensions
{
    public static class MemoryExtensions
    {
        public static Memory<byte> AsBytes<T>(this Memory<T> memory)
            where T : unmanaged;

        public static Memory<TTo> Cast<TFrom, TTo>(this Memory<TFrom> memory)
            where TFrom : unmanaged
            where TTo : unmanaged;
    }

    public static class ReadOnlyMemoryExtensions
    {
        public static ReadOnlyMemory<byte> AsBytes<T>(this ReadOnlyMemory<T> memory)
            where T : unmanaged;

        public static ReadOnlyMemory<TTo> Cast<TFrom, TTo>(this ReadOnlyMemory<TFrom> memory)
            where TFrom : unmanaged
            where TTo : unmanaged;
    }
}
```

## Notes

Marking as draft as this is still being worked on, but feedbacks and reviews are welcome! 😄

## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

Microsoft.Toolkit.HighPerformance/Buffers/Internals/ArrayMemoryManager{TFrom,TTo}.cs

@@ -0,0 +1,134 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Buffers;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Microsoft.Toolkit.HighPerformance.Buffers.Internals.Interfaces;
+using Microsoft.Toolkit.HighPerformance.Extensions;
+using RuntimeHelpers = Microsoft.Toolkit.HighPerformance.Helpers.Internals.RuntimeHelpers;
+
+namespace Microsoft.Toolkit.HighPerformance.Buffers.Internals
+{
+    /// <summary>
+    /// A custom <see cref="MemoryManager{T}"/> that casts data from a <typeparamref name="TFrom"/> array, to <typeparamref name="TTo"/> values.
+    /// </summary>
+    /// <typeparam name="TFrom">The source type of items to read.</typeparam>
+    /// <typeparam name="TTo">The target type to cast the source items to.</typeparam>
+    internal sealed class ArrayMemoryManager<TFrom, TTo> : MemoryManager<TTo>, IMemoryManager
+        where TFrom : unmanaged
+        where TTo : unmanaged
+    {
+        /// <summary>
+        /// The source <typeparamref name="TFrom"/> array to read data from.
+        /// </summary>
+        private readonly TFrom[] array;
+
+        /// <summary>
+        /// The starting offset within <see name="array"/>.
+        /// </summary>
+        private readonly int offset;
+
+        /// <summary>
+        /// The original used length for <see name="array"/>.
+        /// </summary>
+        private readonly int length;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ArrayMemoryManager{TFrom, TTo}"/> class.
+        /// </summary>
+        /// <param name="array">The source <typeparamref name="TFrom"/> array to read data from.</param>
+        /// <param name="offset">The starting offset within <paramref name="array"/>.</param>
+        /// <param name="length">The original used length for <paramref name="array"/>.</param>
+        public ArrayMemoryManager(TFrom[] array, int offset, int length)
+        {
+            this.array = array;
+            this.offset = offset;
+            this.length = length;
+        }
+
+        /// <inheritdoc/>
+        public override Span<TTo> GetSpan()
+        {
+#if SPAN_RUNTIME_SUPPORT
+            ref TFrom r0 = ref this.array.DangerousGetReferenceAt(this.offset);
+            ref TTo r1 = ref Unsafe.As<TFrom, TTo>(ref r0);
+            int length = RuntimeHelpers.ConvertLength<TFrom, TTo>(this.length);
+
+            return MemoryMarshal.CreateSpan(ref r1, length);
+#else
+            Span<TFrom> span = this.array.AsSpan(this.offset, this.length);
+
+            // We rely on MemoryMarshal.Cast here to deal with calculating the effective
+            // size of the new span to return. This will also make the behavior consistent
+            // for users that are both using this type as well as casting spans directly.
+            return MemoryMarshal.Cast<TFrom, TTo>(span);
+#endif
+        }
+
+        /// <inheritdoc/>
+        public override unsafe MemoryHandle Pin(int elementIndex = 0)
+        {
+            if ((uint)elementIndex >= (uint)(this.length * Unsafe.SizeOf<TFrom>() / Unsafe.SizeOf<TTo>()))
+            {
+                ThrowArgumentOutOfRangeExceptionForInvalidIndex();
+            }
+
+            int
+                bytePrefix = this.offset * Unsafe.SizeOf<TFrom>(),
+                byteSuffix = elementIndex * Unsafe.SizeOf<TTo>(),
+                byteOffset = bytePrefix + byteSuffix;
+
+            GCHandle handle = GCHandle.Alloc(this.array, GCHandleType.Pinned);
+
+            ref TFrom r0 = ref this.array.DangerousGetReference();
+            ref byte r1 = ref Unsafe.As<TFrom, byte>(ref r0);
+            ref byte r2 = ref Unsafe.Add(ref r1, byteOffset);
+            void* pi = Unsafe.AsPointer(ref r2);
+
+            return new MemoryHandle(pi, handle);
+        }
+
+        /// <inheritdoc/>
+        public override void Unpin()
+        {
+        }
+
+        /// <inheritdoc/>
+        protected override void Dispose(bool disposing)
+        {
+        }
+
+        /// <inheritdoc/>
+        public Memory<T> GetMemory<T>(int offset, int length)
+            where T : unmanaged
+        {
+            // We need to calculate the right offset and length of the new Memory<T>. The local offset
+            // is the original offset into the wrapped TFrom[] array, while the input offset is the one
+            // with respect to TTo items in the Memory<TTo> instance that is currently being cast.
+            int
+                absoluteOffset = this.offset + RuntimeHelpers.ConvertLength<TTo, TFrom>(offset),
+                absoluteLength = RuntimeHelpers.ConvertLength<TTo, TFrom>(length);
+
+            // We have a special handling in cases where the user is circling back to the original type
+            // of the wrapped array. In this case we can just return a memory wrapping that array directly,
+            // with offset and length being adjusted, without the memory manager indirection.
+            if (typeof(T) == typeof(TFrom))
+            {
+                return (Memory<T>)(object)this.array.AsMemory(absoluteOffset, absoluteLength);
+            }
+
+            return new ArrayMemoryManager<TFrom, T>(this.array, absoluteOffset, absoluteLength).Memory;
+        }
+
+        /// <summary>
+        /// Throws an <see cref="ArgumentOutOfRangeException"/> when the target index for <see cref="Pin"/> is invalid.
+        /// </summary>
+        private static void ThrowArgumentOutOfRangeExceptionForInvalidIndex()
+        {
+            throw new ArgumentOutOfRangeException("elementIndex", "The input index is not in the valid range");
+        }
+    }
+}

Microsoft.Toolkit.HighPerformance/Buffers/Internals/Interfaces/IMemoryManager.cs

@@ -0,0 +1,25 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Buffers;
+
+namespace Microsoft.Toolkit.HighPerformance.Buffers.Internals.Interfaces
+{
+    /// <summary>
+    /// An interface for a <see cref="MemoryManager{T}"/> instance that can reinterpret its underlying data.
+    /// </summary>
+    internal interface IMemoryManager
+    {
+        /// <summary>
+        /// Creates a new <see cref="Memory{T}"/> that reinterprets the underlying data for the current instance.
+        /// </summary>
+        /// <typeparam name="T">The target type to cast the items to.</typeparam>
+        /// <param name="offset">The starting offset within the data store.</param>
+        /// <param name="length">The original used length for the data store.</param>
+        /// <returns>A new <see cref="Memory{T}"/> instance of the specified type, reinterpreting the current items.</returns>
+        Memory<T> GetMemory<T>(int offset, int length)
+            where T : unmanaged;
+    }
+}

Microsoft.Toolkit.HighPerformance/Buffers/Internals/ProxyMemoryManager{TFrom,TTo}.cs

@@ -0,0 +1,134 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Buffers;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Microsoft.Toolkit.HighPerformance.Buffers.Internals.Interfaces;
+using RuntimeHelpers = Microsoft.Toolkit.HighPerformance.Helpers.Internals.RuntimeHelpers;
+
+namespace Microsoft.Toolkit.HighPerformance.Buffers.Internals
+{
+    /// <summary>
+    /// A custom <see cref="MemoryManager{T}"/> that casts data from a <see cref="MemoryManager{T}"/> of <typeparamref name="TFrom"/>, to <typeparamref name="TTo"/> values.
+    /// </summary>
+    /// <typeparam name="TFrom">The source type of items to read.</typeparam>
+    /// <typeparam name="TTo">The target type to cast the source items to.</typeparam>
+    internal sealed class ProxyMemoryManager<TFrom, TTo> : MemoryManager<TTo>, IMemoryManager
+        where TFrom : unmanaged
+        where TTo : unmanaged
+    {
+        /// <summary>
+        /// The source <see cref="MemoryManager{T}"/> to read data from.
+        /// </summary>
+        private readonly MemoryManager<TFrom> memoryManager;
+
+        /// <summary>
+        /// The starting offset within <see name="memoryManager"/>.
+        /// </summary>
+        private readonly int offset;
+
+        /// <summary>
+        /// The original used length for <see name="memoryManager"/>.
+        /// </summary>
+        private readonly int length;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ProxyMemoryManager{TFrom, TTo}"/> class.
+        /// </summary>
+        /// <param name="memoryManager">The source <see cref="MemoryManager{T}"/> to read data from.</param>
+        /// <param name="offset">The starting offset within <paramref name="memoryManager"/>.</param>
+        /// <param name="length">The original used length for <paramref name="memoryManager"/>.</param>
+        public ProxyMemoryManager(MemoryManager<TFrom> memoryManager, int offset, int length)
+        {
+            this.memoryManager = memoryManager;
+            this.offset = offset;
+            this.length = length;
+        }
+
+        /// <inheritdoc/>
+        public override Span<TTo> GetSpan()
+        {
+            Span<TFrom> span = this.memoryManager.GetSpan().Slice(this.offset, this.length);
+
+            return MemoryMarshal.Cast<TFrom, TTo>(span);
+        }
+
+        /// <inheritdoc/>
+        public override MemoryHandle Pin(int elementIndex = 0)
+        {
+            if ((uint)elementIndex >= (uint)(this.length * Unsafe.SizeOf<TFrom>() / Unsafe.SizeOf<TTo>()))
+            {
+                ThrowArgumentExceptionForInvalidIndex();
+            }
+
+            int
+                bytePrefix = this.offset * Unsafe.SizeOf<TFrom>(),
+                byteSuffix = elementIndex * Unsafe.SizeOf<TTo>(),
+                byteOffset = bytePrefix + byteSuffix;
+
+#if NETSTANDARD1_4
+            int
+                shiftedOffset = byteOffset / Unsafe.SizeOf<TFrom>(),
+                remainder = byteOffset - (shiftedOffset * Unsafe.SizeOf<TFrom>());
+#else
+            int shiftedOffset = Math.DivRem(byteOffset, Unsafe.SizeOf<TFrom>(), out int remainder);
+#endif
+
+            if (remainder != 0)
+            {
+                ThrowArgumentExceptionForInvalidAlignment();
+            }
+
+            return this.memoryManager.Pin(shiftedOffset);
+        }
+
+        /// <inheritdoc/>
+        public override void Unpin()
+        {
+            this.memoryManager.Unpin();
+        }
+
+        /// <inheritdoc/>
+        protected override void Dispose(bool disposing)
+        {
+            ((IDisposable)this.memoryManager).Dispose();
+        }
+
+        /// <inheritdoc/>
+        public Memory<T> GetMemory<T>(int offset, int length)
+            where T : unmanaged
+        {
+            // Like in the other memory manager, calculate the absolute offset and length
+            int
+                absoluteOffset = this.offset + RuntimeHelpers.ConvertLength<TTo, TFrom>(offset),
+                absoluteLength = RuntimeHelpers.ConvertLength<TTo, TFrom>(length);
+
+            // Skip one indirection level and slice the original memory manager, if possible
+            if (typeof(T) == typeof(TFrom))
+            {
+                return (Memory<T>)(object)this.memoryManager.Memory.Slice(absoluteOffset, absoluteLength);
+            }
+
+            return new ProxyMemoryManager<TFrom, T>(this.memoryManager, absoluteOffset, absoluteLength).Memory;
+        }
+
+        /// <summary>
+        /// Throws an <see cref="ArgumentOutOfRangeException"/> when the target index for <see cref="Pin"/> is invalid.
+        /// </summary>
+        private static void ThrowArgumentExceptionForInvalidIndex()
+        {
+            throw new ArgumentOutOfRangeException("elementIndex", "The input index is not in the valid range");
+        }
+
+        /// <summary>
+        /// Throws an <see cref="ArgumentOutOfRangeException"/> when <see cref="Pin"/> receives an invalid target index.
+        /// </summary>
+        private static void ThrowArgumentExceptionForInvalidAlignment()
+        {
+            throw new ArgumentOutOfRangeException("elementIndex", "The input index doesn't result in an aligned item access");
+        }
+    }
+}