Add non-allocating mul_mod

[recmo]

Motivation

The Vec allocation for intermediate result adds a lot of overhead to mul_mod.

Solution

Unfortunately no good solutions, but in the new variant the caller can provide
a compile time hint.

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (8089eab) 80.48% compared to head (9f2a4be) 80.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
- Coverage   80.48%   80.17%   -0.32%     
==========================================
  Files          54       54              
  Lines        6140     6153      +13     
==========================================
- Hits         4942     4933       -9     
- Misses       1198     1220      +22     

Files	Coverage Δ
src/modular.rs	92.63% <0.00%> (-6.81%)	⬇️

... and 3 files with indirect coverage changes

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[DaniPopes]

@prestwich this can be closed with #373