Allow passing ReadOnlySpan<byte> to Marshal.Copy

[sweemer]

Not sure whether this repo is the right place to report this, but I recently discovered that Marshal.Copy only takes byte[] and not ReadOnlySpan<byte>. Any reason the former can't be allowed as well?

I know I can just call ToArray() on the span to pass it, but the extra copy to a reference type might not always be desirable.

using System;
using System.Runtime.InteropServices;

public class C {
    public void M() {
        var bytes = new byte[] { 0, 1 };
        var ptr = Marshal.AllocHGlobal(bytes.Length);
        var span = new ReadOnlySpan<byte>(bytes);
        Marshal.Copy(bytes, 0, ptr, bytes.Length);
        Marshal.Copy(span, 0, ptr, bytes.Length); // error CS1503: Argument 1: cannot convert from 'System.ReadOnlySpan<byte>' to 'byte[]'
    }
}

[colejohnson66]

API requests go in @dotnet/runtime.