Implement signed modulo

[mborland]

Closes: #129

[codecov]

Codecov Report

❌ Patch coverage is 90.72848% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.8%. Comparing base (b5a1a28) to head (4e21abd).
⚠️ Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
...boost/safe_numbers/detail/signed_integer_basis.hpp	75.0%	10 Missing ⚠️
test/test_signed_modulo.cpp	96.4%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #162     +/-   ##
=========================================
- Coverage     96.9%   96.8%   -0.1%     
=========================================
  Files          119     120      +1     
  Lines         9109    9260    +151     
  Branches      3446    3507     +61     
=========================================
+ Hits          8825    8962    +137     
- Misses         218     232     +14     
  Partials        66      66             

Files with missing lines	Coverage Δ
test/test_signed_modulo.cpp	96.4% <96.4%> (ø)
...boost/safe_numbers/detail/signed_integer_basis.hpp	72.7% <75.0%> (+0.3%)	⬆️

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