Add support for string interning

[AArnott]

Closes #143

[codecov]

Codecov Report

Attention: Patch coverage is 72.41379% with 16 lines in your changes missing coverage. Please review.

Project coverage is 76.50%. Comparing base (1851e9b) to head (e51ac7b).

Files with missing lines	Patch %	Lines
...bank.MessagePack/Converters/PrimitiveConverters.cs	33.33%	9 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
- Coverage   76.60%   76.50%   -0.10%     
==========================================
  Files         139      140       +1     
  Lines        9898     9956      +58     
  Branches     1374     1385      +11     
==========================================
+ Hits         7582     7617      +35     
- Misses       1912     1927      +15     
- Partials      404      412       +8     

Flag	Coverage Δ
Linux	76.50% <72.41%> (-0.10%)	⬇️

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