arithmetic internals: Rewrite limbs_equal_limb_constant_time in Rust.

[briansmith]

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[codecov]

Codecov Report

Attention: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.73%. Comparing base (91d81df) to head (7280c5a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/limb.rs	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2280      +/-   ##
==========================================
- Coverage   96.73%   96.73%   -0.01%     
==========================================
  Files         171      171              
  Lines       20871    20859      -12     
  Branches      488      487       -1     
==========================================
- Hits        20189    20177      -12     
  Misses        577      577              
  Partials      105      105              

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