Add floor_root_vartime, checked_root_vartime for Uint

[andrewwhitehead]

Related to #1082

[codecov]

Codecov Report

❌ Patch coverage is 98.38710% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.45%. Comparing base (4809559) to head (96ffded).

Files with missing lines	Patch %	Lines
src/uint/root.rs	97.29%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1089      +/-   ##
==========================================
+ Coverage   79.26%   79.45%   +0.18%     
==========================================
  Files         176      177       +1     
  Lines       18021    18143     +122     
==========================================
+ Hits        14285    14415     +130     
+ Misses       3736     3728       -8     

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