Add a reference to MessagePack implementation

[AArnott]

This pull request includes updates to the README.md file, enhancements to the SpanDictionary class, and improvements to the equality members of the BaseClass and DerivedClass in the test types.

Documentation Updates:

• README.md: Added a new section listing known libraries based on TypeShape, including Nerdbank.MessagePack.

Code Enhancements:

• src/TypeShape.Examples/Utilities/SpanDictionary.cs: Added a new overload for the ToSpanDictionary method to support mapping with a value selector.

Test Improvements:

• tests/TypeShape.Tests/TestTypes.cs: Implemented IEquatable<T> for BaseClass and DerivedClass, and added appropriate Equals and GetHashCode methods.

[AArnott]

Incidentally, upgrading the baseline from MessagePack v2 to v3 produces this:

Serialization

Method	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
Serialize_TypeShape	130.9 ns	0.37 ns	0.33 ns	0.95	-	NA
Serialize_Library	137.6 ns	0.23 ns	0.21 ns	1.00	-	NA

This improves serialization slightly, making TypeShape fair slightly worse by comparison.

Deserialization

Method	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
Deserialize_TypeShape	148.1 ns	0.63 ns	0.56 ns	1.21	32 B	1.00
Deserialize_Library	122.2 ns	0.74 ns	0.61 ns	1.00	32 B	1.00

Deserialization improves significantly, making TypeShape look quite a bit worse. This is due to using a dictionary to look up property names, which the AOT formatter avoids.

[AArnott]

Serialization is somehow faster than the Ref.Emit specialized code produced by the MessagePack library. I don't know how that's possible.

I suspect it's because in this version, we call MessagePackWriter.Write(string) directly rather than including a level of indirection that allows for some other serializer to hijack that operation at runtime to serialize something else (e.g. so that a string is only serialized once).

[AArnott]

@neuecc this should interest you.