Merge pull request MessagePack-CSharp#52 from AArnott/fix50

Avoid allocating many small arrays while serializing

Merge pull request MessagePack-CSharp#27 from AArnott/fix9

Add support for IBufferWriter<byte>

Remove accidentally checked in graph2.xlsx

Add benchmarks for array, span, and string pinning

Repro state for feedback ticket https://developercommunity.visualstud…

…io.com/content/problem/414429/error-list-full-of-errors-from-the-previous-soluti.html