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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