Improve 32-bit addition implementation

[mborland]

Removes a few branches and swaps from the existing implementation
Closes: #687

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.1%. Comparing base (5719f40) to head (2a2c8f9).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #689     +/-   ##
=========================================
- Coverage     99.2%   99.1%   -0.0%     
=========================================
  Files          230     230             
  Lines        16407   16386     -21     
  Branches      1791    1781     -10     
=========================================
- Hits         16262   16238     -24     
- Misses         145     148      +3     

Files	Coverage Δ
include/boost/decimal/decimal32.hpp	99.8% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal32_fast.hpp	97.2% <100.0%> (-0.1%)	⬇️
include/boost/decimal/detail/add_impl.hpp	100.0% <100.0%> (ø)
test/test_decimal32.cpp	100.0% <100.0%> (ø)

... and 3 files with indirect coverage changes

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