Update comparison methods for UintRef

[andrewwhitehead]

UintRef was deriving PartialEq, which simply compared the limb slices and was not constant-time. This makes the PartialEq implementation use CtEq, consistent with Uint and BoxedUint. It also adds PartialOrd and Ord implementations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.99%. Comparing base (2021752) to head (87073e3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1311   +/-   ##
=======================================
  Coverage   90.99%   90.99%           
=======================================
  Files         189      189           
  Lines       22560    22573   +13     
=======================================
+ Hits        20528    20541   +13     
  Misses       2032     2032           

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