Faster floor_sqrt, checked_sqrt

[andrewwhitehead]

• An algorithm which scales better for large integers is used (approx O(2n)) - this is algorithm 1.12 in Modern Computer Arithmetic rather than 1.13
• Addition and subtraction methods in UintRef are adjusted to accept an rhs smaller than self. This was inconsistently applied before
• Performance is much better, about 70% improved for U256 and 95% for a 4096-bit BoxedUint
• checked_sqrt no longer performs an additional squaring, so its performance matches floor_sqrt

[codecov]

Codecov Report

❌ Patch coverage is 98.95833% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.91%. Comparing base (ecf51af) to head (69f4ab6).

Files with missing lines	Patch %	Lines
src/uint/ref_type/sqrt.rs	98.32%	3 Missing ⚠️
src/uint/sqrt.rs	97.22%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1258      +/-   ##
==========================================
+ Coverage   90.84%   90.91%   +0.06%     
==========================================
  Files         186      187       +1     
  Lines       21673    21937     +264     
==========================================
+ Hits        19688    19943     +255     
- Misses       1985     1994       +9     

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