Reduce string encoding/decoding in MessagePackFormatter

[AArnott]

Several strings are very common in JSON-RPC. Rather than encode/decode them repeatedly, allocating new strings each time we decode, we instead use pre-encoded bytes and recognize the encoded bytes without decoding them.

The techniques used here come from MessagePack's own mpc-tool code output. So even though we're using MessagePack's "internal" namespace, it needs to be stable within a major version to keep mpc generated code working.

• Sample deserialization decoder optimization
• Sample serialization encoder optimization

I recognize that this compromises readability, but perf traces indicate that the decoding time and allocation costs are significant, so I believe it is justified. I define constants and add code comments to help explain it.

Also note (because I only learned this recently) that in C# the syntax below produces very efficient code. It never allocates an array -- ever. Not even once. Read the IL yourself. This syntax leads the C# compiler to generate code that initializes the ReadOnlySpan<byte> to point directly into the binary of the loaded assembly itself to get at the bytes in the compiled binary.

static ReadOnlySpan<byte> VersionPropertyNameBytes => new byte[] { 167, 106, 115, 111, 110, 114, 112, 99 };

I did not optimize every single formatter in the MessagePackFormatter.cs file. Just the hot path ones. We can optimize the error formatter if that becomes important.

[codecov-io]

Codecov Report

Merging #594 into master will decrease coverage by 0.02%.
The diff coverage is 94.56%.

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
- Coverage   89.93%   89.91%   -0.03%     
==========================================
  Files          54       54              
  Lines        4492     4522      +30     
==========================================
+ Hits         4040     4066      +26     
- Misses        452      456       +4     

Impacted Files	Coverage Δ
src/StreamJsonRpc/MessagePackFormatter.cs	91.56% <94.56%> (-0.21%)	⬇️

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[BertanAygun]

Approving based on comments