Add modular exponentiation function

[mborland]

Closes: #394

[cppalliance-bot]

An automated preview of the documentation is available at https://399.int128.prtest3.cppalliance.org/libs/int128/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-05-12 14:19:05 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.10%. Comparing base (a793f10) to head (56c0039).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #399      +/-   ##
===========================================
+ Coverage    95.99%   96.10%   +0.10%     
===========================================
  Files           69       71       +2     
  Lines         5948     6109     +161     
  Branches      1972     2045      +73     
===========================================
+ Hits          5710     5871     +161     
  Misses          34       34              
  Partials       204      204              

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