fix: better document and isolate performance hack

[fredbi]

• fixes Isolate unsafe.Sizeof in a wrapper in a dedicated file #110

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.07%. Comparing base (d91832e) to head (db93abe).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   89.09%   89.07%   -0.02%     
==========================================
  Files          25       26       +1     
  Lines        1706     1703       -3     
==========================================
- Hits         1520     1517       -3     
  Misses        150      150              
  Partials       36       36              

Flag	Coverage Δ
oldstable-macos-latest	87.72% <100.00%> (-0.03%)	⬇️
oldstable-ubuntu-latest	87.72% <100.00%> (-0.03%)	⬇️
oldstable-windows-latest	88.72% <100.00%> (-0.02%)	⬇️
stable-macos-latest	87.72% <100.00%> (-0.03%)	⬇️
stable-ubuntu-latest	87.72% <100.00%> (-0.03%)	⬇️
stable-windows-latest	88.72% <100.00%> (-0.02%)	⬇️

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