Add support for exact division

[andrewwhitehead]

This adds div_exact and div_exact_vartime methods for Uint, BoxedUint, and Limb. There is currently no trait support.

For cases where it applies, exact division can be much faster than div/rem. In my tests this is about 40% faster for a 4096-bit BoxedUint.

The algorithm is similar to DivideByWord in Modern Computer Arithmetic but I don't have a reference for the adjustments to support a multi-word divisor.

[codecov]

Codecov Report

❌ Patch coverage is 99.34211% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.91%. Comparing base (aa41fdb) to head (eecaff2).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/uint/ref_type/div.rs	98.82%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1256      +/-   ##
==========================================
+ Coverage   90.84%   90.91%   +0.07%     
==========================================
  Files         186      186              
  Lines       21673    21823     +150     
==========================================
+ Hits        19688    19841     +153     
+ Misses       1985     1982       -3     

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[andrewwhitehead]

Oops, I need to add a 32-bit version of the limb inversion. I have a fix but it depends on #1257 which is a separate issue.