Document bitpar counting

[jakobnissen]

Also add some inbounds annotations. These were exported, but had no explicit docstrings.
Future work could include cleaning up the bitpar compiler to emit more efficient code, and removing the weird methods.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.48%. Comparing base (95d9218) to head (9efbbe9).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
- Coverage   90.87%   90.48%   -0.39%     
==========================================
  Files          31       31              
  Lines        2400     2397       -3     
==========================================
- Hits         2181     2169      -12     
- Misses        219      228       +9     

Flag	Coverage Δ
unittests	90.48% <100.00%> (-0.39%)	⬇️

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