Correct divide-by-zero in cpp_int modulus operations.

[jzmaddock]

Also added some more efficient operator overloads for the trivial case. Fixes #624

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.1%. Comparing base (f9fdcc2) to head (b0ea443).

❗ Current head b0ea443 differs from pull request most recent head 891e2c3

Please upload reports for the commit 891e2c3 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #625     +/-   ##
=========================================
+ Coverage     94.1%   94.1%   +0.1%     
=========================================
  Files          274     275      +1     
  Lines        26766   26794     +28     
=========================================
+ Hits         25161   25188     +27     
- Misses        1605    1606      +1     

Files	Coverage Δ
test/git_issue_624.cpp	100.0% <100.0%> (ø)
include/boost/multiprecision/cpp_int/divide.hpp	92.5% <83.4%> (-<0.1%)	⬇️

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