support serializing i128 / u128

[davidhewitt]

Fixes #111

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.64%. Comparing base (991f3f5) to head (a2ba6a9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
+ Coverage   85.58%   85.64%   +0.06%     
==========================================
  Files           3        3              
  Lines        1193     1198       +5     
  Branches     1193     1198       +5     
==========================================
+ Hits         1021     1026       +5     
  Misses        106      106              
  Partials       66       66              

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