Improve async deserialization performance

[AArnott]

• Improve string deserialization perf overall (from DeserializeAsync methods)
• Opportunistically use synchronous deserialization during async enumeration of top-level elements when we have the next item buffered already anyway.
• Use sync deserialization opportunistically during async enumeration with envelopes

Together, this gets the async top-level enumeration scenario from 38% slower than MessagePack-CSharp to 14% faster.

Closes #253

[codecov]

Codecov Report

Attention: Patch coverage is 88.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 84.49%. Comparing base (56f0454) to head (d2ba765).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Nerdbank.MessagePack/MessagePackSerializer.cs	76.92%	3 Missing ⚠️
...rc/Nerdbank.MessagePack/StreamingDeserializer`1.cs	86.36%	3 Missing ⚠️
src/Nerdbank.MessagePack/MessagePackAsyncReader.cs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
- Coverage   84.53%   84.49%   -0.05%     
==========================================
  Files         167      167              
  Lines       13663    13704      +41     
  Branches     1458     1459       +1     
==========================================
+ Hits        11550    11579      +29     
- Misses       2085     2097      +12     
  Partials       28       28              

Flag	Coverage Δ
Linux	82.88% <88.33%> (+0.03%)	⬆️
Windows	84.47% <88.33%> (-0.02%)	⬇️

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Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (3)

test/Nerdbank.MessagePack.Tests/StreamingEnumerableTests.cs:100

• Ensure that the new test case adequately covers the behavior of opportunistically using synchronous deserialization during async enumeration.

[Fact]

src/Nerdbank.MessagePack/MessagePackAsyncReader.cs:224

• Ensure that the new method GetBufferedStructuresCount is covered by tests to verify its correctness.

internal int GetBufferedStructuresCount(int countUpTo, SerializationContext context, out bool reachedMaxCount)

src/Nerdbank.MessagePack/Converters/PrimitiveConverters.cs:68

• The word 'numbe' should be 'number'.

// We'll always require at least a reasonable numbe of bytes to decode at once,