Make vartime bit shifts return an Option instead of CtOption

[andrewwhitehead]

Related to #1046

This also adjusts the ShlVartime/ShrVartime trait implementations to use the vartime implementations. They mostly weren't before?

[codecov]

Codecov Report

❌ Patch coverage is 85.47486% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.54%. Comparing base (a54858d) to head (74c4fd4).

Files with missing lines	Patch %	Lines
src/uint/boxed/shr.rs	22.22%	14 Missing ⚠️
src/uint/boxed/shl.rs	44.44%	10 Missing ⚠️
src/uint/shl.rs	97.61%	1 Missing ⚠️
src/uint/shr.rs	97.56%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1096      +/-   ##
==========================================
+ Coverage   79.34%   79.54%   +0.19%     
==========================================
  Files         176      176              
  Lines       18058    18031      -27     
==========================================
+ Hits        14328    14342      +14     
+ Misses       3730     3689      -41     

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