Fixes for exact division edge cases

[andrewwhitehead]

This adds fixes for a couple types of inputs that were not producing the correct result:

• Zero dividends should always divide exactly, even when the divisor has more limbs
• Inexact division could be undetected when the dividend had one or more zero-valued lower limbs

The suite of proptests for BoxedUint are updated to explicitly create inputs with a single bit set, only high bits set, and only low bits set (which also detects these issues). Some tests are expanded to test differently-sized inputs where this was not being exercised before.

[andrewwhitehead]

Changing this highlights the issue fixed by #1291

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.03%. Comparing base (2021752) to head (65f40c8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1310      +/-   ##
==========================================
+ Coverage   90.99%   91.03%   +0.04%     
==========================================
  Files         189      189              
  Lines       22560    22585      +25     
==========================================
+ Hits        20528    20561      +33     
+ Misses       2032     2024       -8     

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[andrewwhitehead]

This does make the test runs significantly slower at the moment, I'll look at reducing the frequency of tests for larger BoxedUints.