bigint: Enforce that Mont storage is in the correct form.

[briansmith]

In the future, we may rely on these checks for safety; start acting like we already do.

[codecov]

Codecov Report

❌ Patch coverage is 77.14286% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.44%. Comparing base (bfbeca5) to head (ddc5ff5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/arithmetic/bigint/modulus/mont.rs	74.19%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2855      +/-   ##
==========================================
- Coverage   96.47%   96.44%   -0.04%     
==========================================
  Files         203      203              
  Lines       20354    20376      +22     
  Branches      519      523       +4     
==========================================
+ Hits        19637    19652      +15     
- Misses        597      601       +4     
- Partials      120      123       +3     

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