feat: make add and sub const fns

[DaniPopes]

Motivation

By changing for with while we can make mark add and sub functions as const.

Solution

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (706ffd0) 80.63% compared to head (038ac4c) 80.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
- Coverage   80.63%   80.49%   -0.14%     
==========================================
  Files          54       54              
  Lines        6036     6040       +4     
==========================================
- Hits         4867     4862       -5     
- Misses       1169     1178       +9     

Files	Coverage Δ
src/cmp.rs	100.00% <ø> (ø)
src/add.rs	74.82% <81.81%> (+0.72%)	⬆️

... and 5 files with indirect coverage changes

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