Make vartime inversion return an Option instead of CtOption

[andrewwhitehead]

Related to #1046

[codecov]

Codecov Report

❌ Patch coverage is 45.45455% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.31%. Comparing base (a54858d) to head (83f945e).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/modular/safegcd/boxed.rs	44.00%	14 Missing ⚠️
src/modular/boxed_monty_form/invert.rs	16.66%	10 Missing ⚠️
src/modular/const_monty_form/invert.rs	0.00%	10 Missing ⚠️
src/modular/monty_form/invert.rs	10.00%	9 Missing ⚠️
src/modular/safegcd.rs	78.57%	6 Missing ⚠️
src/traits.rs	0.00%	2 Missing ⚠️
src/uint/boxed/invert_mod.rs	33.33%	2 Missing ⚠️
src/uint.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1097      +/-   ##
==========================================
- Coverage   79.34%   79.31%   -0.04%     
==========================================
  Files         176      176              
  Lines       18058    18074      +16     
==========================================
+ Hits        14328    14335       +7     
- Misses       3730     3739       +9     

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