refactor: optimize coefficient extraction to avoid recursion limits

[tlambert03]

another attempt to fix #64

this uses an explicit stack rather than recursion

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.45%. Comparing base (0fdfae7) to head (de15fe8).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   75.72%   83.45%   +7.73%     
==========================================
  Files          17       11       -6     
  Lines         828      810      -18     
  Branches      154        0     -154     
==========================================
+ Hits          627      676      +49     
+ Misses        136      134       -2     
+ Partials       65        0      -65     

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