Update NetDataReader, NetDataWriter & FastBitConverter + add docs

[Lacyway]

This PR does 3 things:

• Update both NetDataReader & NetDataWriter to use faster methods of reading/writing data
• Update FastBitConverter to use WriteUnaligned<T>()
• Updates all methods/properties in each class with markup documentation

These methods have been used in Project Fika for a very long time without issue, and as such I have decided to PR them per our discussion on Discord.

No signatures should be changed, meaning users do not have to update anything on their end.

I have included conditional compilation directives where >.NET 8 uses the Unsafe class, and netstandard2.1 (mainly for Unity) uses fixed, which is slower, but does not require external libraries. The difference is so small that it does not warrant the requirement of importing this library:

I also changed to Unsafe.WriteUnaligned<T>() in FastBitConverter for >.NET 8 as it is faster when writing unaligned (see benchmark below)

I have ran all tests and they are all successful. I also updated the tests to include some of the new methods, e.g. for Unmanaged<T> and Enum. I am unsure how to test this for netstandard2.1 as there are no included tests for it.

Please let me know if you'd like any changes or if there are any questions.