Optimize add to avoid overflow check

[schouhy]

Optimize add to avoid overflow check

Type of change

• Optimization

Checklist

• Linked to Github Issue
• Unit tests added
• This change requires new documentation.
  • Documentation has been added/updated.
• This change is an Optimization
  • Benchmarks added/run

[codecov-commenter]

Codecov Report

Merging #520 (b04d4c9) into main (d95c83c) will decrease coverage by 0.04%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main     #520      +/-   ##
==========================================
- Coverage   94.51%   94.48%   -0.04%     
==========================================
  Files          62       62              
  Lines        8949     8951       +2     
==========================================
- Hits         8458     8457       -1     
- Misses        491      494       +3     

Files Changed	Coverage Δ
...src/field/fields/montgomery_backed_prime_fields.rs	93.16% <80.00%> (-0.39%)	⬇️

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[Oppen]

Two suggestions:

1. Make modulus_has_one_spare_bit #[inline(always)];
2. Evaluate to a const before the if guard, so we can be sure it's always evaluated at compile time.
   Point 2 is because a const fn is not intrinsically always evaluated as a constant, but it signals that it can be used to create constants. By storing the result into a const you guarantee that, so then the correct branch is known at compile-time to always be taken and thus the check and the other branch get removed by the compiler.