Add Nat::divmod

[johnynek]

just another exercise.

With Nat, linear depth recursion will overflow the stack, so the #1172 would be very useful.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.57%. Comparing base (04a0e87) to head (bc6a3b4).
Report is 22 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1192      +/-   ##
==========================================
+ Coverage   91.32%   91.57%   +0.25%     
==========================================
  Files          96       96              
  Lines       11846    12058     +212     
  Branches     2675     2744      +69     
==========================================
+ Hits        10818    11042     +224     
+ Misses       1028     1016      -12     

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