Implement signed subtraction

[mborland]

Closes: #126

[codecov]

Codecov Report

❌ Patch coverage is 86.89320% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.5%. Comparing base (c38b85b) to head (adc23e2).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
...boost/safe_numbers/detail/signed_integer_basis.hpp	70.0%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #158     +/-   ##
=========================================
- Coverage     97.7%   97.5%   -0.2%     
=========================================
  Files          116     117      +1     
  Lines         8497    8703    +206     
  Branches      3219    3294     +75     
=========================================
+ Hits          8300    8479    +179     
- Misses         131     158     +27     
  Partials        66      66             

Files with missing lines	Coverage Δ
test/test_signed_subtraction.cpp	100.0% <100.0%> (ø)
...boost/safe_numbers/detail/signed_integer_basis.hpp	75.8% <70.0%> (-3.7%)	⬇️

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