Clarity Value toString function

[reedrosenbluth]

This PR adds a function to convert TS Clarity Values to their string (source code) representation so they can be displayed easily. This is important for the Explorer.

relevant sidecar issue: https://github.com/blockstack/stacks-blockchain-sidecar/issues/99

Type of Change

• [x] New feature
• [ ] Bug fix
• [ ] API reference/documentation update
• [ ] Other

Checklist

• [x] Code is commented where needed
• [x] Unit test coverage for new or modified code paths
• [x] npm run test passes
• [x] Changelog is updated
• [x] Tag 1 of @yknl, @zone117x, @reedrosenbluth for review

[codecov[bot]]

Codecov Report

Merging #83 into master will increase coverage by 0.33%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
+ Coverage   83.91%   84.24%   +0.33%     
==========================================
  Files          27       27              
  Lines        1666     1701      +35     
  Branches      350      366      +16     
==========================================
+ Hits         1398     1433      +35     
  Misses        266      266              
  Partials        2        2              

Impacted Files	Coverage Δ
src/clarity/clarityValue.ts	100.00% <100.00%> (ø)
src/clarity/index.ts	100.00% <100.00%> (ø)

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