arithmetic/limbs internals: Clarify limbs < limbs checks.

[briansmith]

Reject empty or non-matching-length inputs. Refactor the internal API so that it is clearer that we're nearly always using this function to reject too-large inputs. This makes it easier to see that it is OK to return an error when previously we didn't.

This isn't intended to have any effect other than making the code easier to audit.

[codecov]

Codecov Report

Attention: Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.75%. Comparing base (1371308) to head (b50f143).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/arithmetic/bigint/modulusvalue.rs	50.00%	0 Missing and 2 partials ⚠️
src/limb.rs	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2278      +/-   ##
==========================================
- Coverage   96.75%   96.75%   -0.01%     
==========================================
  Files         171      171              
  Lines       20868    20875       +7     
  Branches      491      489       -2     
==========================================
+ Hits        20191    20197       +6     
- Misses        571      572       +1     
  Partials      106      106              

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