fix: serialize uint strings with literal u prefix

[reedrosenbluth]

Description

This PR introduces a slight change to the cvToString() function. Uints are now serialized like "u10" instead of like "10".

Issue: https://github.com/blockstack/stacks-blockchain/issues/1699

Type of Change

• New feature
• Bug fix
• API reference/documentation update
• Other

Does this introduce a breaking change?

No

Checklist

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

[codecov]

Codecov Report

Merging #98 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #98   +/-   ##
=======================================
  Coverage   84.01%   84.01%           
=======================================
  Files          28       28           
  Lines        1802     1802           
  Branches      367      367           
=======================================
  Hits         1514     1514           
  Misses        286      286           
  Partials        2        2           

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

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[yknl]

LGTM