Implement basis of signed integers and basic addition

[mborland]

Closes: #125

[codecov]

Codecov Report

❌ Patch coverage is 95.30435% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.6%. Comparing base (57662c5) to head (7c4f8f1).
⚠️ Report is 25 commits behind head on develop.

Files with missing lines	Patch %	Lines
...boost/safe_numbers/detail/signed_integer_basis.hpp	74.1%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #132      +/-   ##
==========================================
+ Coverage     67.1%   97.6%   +30.5%     
==========================================
  Files          106     112       +6     
  Lines         7217    7858     +641     
  Branches      2684    2904     +220     
==========================================
+ Hits          4839    7662    +2823     
- Misses         107     130      +23     
+ Partials      2271      66    -2205     

Files with missing lines	Coverage Δ
include/boost/safe_numbers/detail/type_traits.hpp	100.0% <ø> (ø)
...ost/safe_numbers/detail/unsigned_integer_basis.hpp	86.2% <100.0%> (+7.5%)	⬆️
include/boost/safe_numbers/limits.hpp	100.0% <ø> (+20.0%)	⬆️
test/test_signed_addition.cpp	100.0% <100.0%> (ø)
test/test_signed_comparisons.cpp	100.0% <100.0%> (ø)
test/test_signed_construction.cpp	100.0% <100.0%> (ø)
test/test_signed_conversions.cpp	100.0% <100.0%> (ø)
test/test_signed_unary_operators.cpp	100.0% <100.0%> (ø)
...boost/safe_numbers/detail/signed_integer_basis.hpp	74.1% <74.1%> (ø)

... and 85 files with indirect coverage changes

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