arithmetic: Encapsulate how elements get re-encoded in Montgomery form.

[briansmith]

Instead of having the user multiply by an element with the value 1RR (or 1RRR), just let them ask for what they want, and then do it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.66%. Comparing base (e9f69ec) to head (7eecfbd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2731      +/-   ##
==========================================
- Coverage   96.66%   96.66%   -0.01%     
==========================================
  Files         198      198              
  Lines       20196    20187       -9     
  Branches      521      521              
==========================================
- Hits        19523    19513      -10     
  Misses        564      564              
- Partials      109      110       +1     

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