Return Option from vartime inversion and pow methods

[cong-or]

#1096 converted the vartime bit shift methods from CtOption to Option. This does the same for vartime inversion and pow.

Since these methods are already variable-time, wrapping results in CtOption doesn't add any constant-time guarantees — it just forces eager evaluation in combinators where Option would
be lazy.

The Invert trait impls still return CtOption — #1097 tried changing that and got reverted in #1124 because elliptic-curve needs Invert::Output to stay flexible. So the trait impls
convert back from Option via match.

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.83%. Comparing base (40205f1) to head (6352578).

Files with missing lines	Patch %	Lines
src/modular/const_monty_form/invert.rs	0.00%	11 Missing ⚠️
src/uint/boxed/invert_mod.rs	0.00%	2 Missing ⚠️
src/uint/invert_mod.rs	80.00%	2 Missing ⚠️
src/modular/fixed_monty_form/invert.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1260      +/-   ##
==========================================
- Coverage   90.84%   90.83%   -0.01%     
==========================================
  Files         186      186              
  Lines       21673    21686      +13     
==========================================
+ Hits        19688    19699      +11     
- Misses       1985     1987       +2     

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[tarcieri]

I'm a little confused that cargo semver-checks isn't complaining, but these are breaking changes and we're in the stable part of a release cycle.

[cong-or]

I'm a little confused that cargo semver-checks isn't complaining, but these are breaking changes and we're in the stable part of a release cycle.

@tarcieri No worries — happy to rebase this when you're ready to collect breaking changes for the next release.