Refactor decimal128 and decimal128_fast

[mborland]

Closes: #678
Closes: #679
Closes: #680
Closes: #681
Closes: #618

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.0%. Comparing base (8d6d270) to head (ba77d62).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #683     +/-   ##
=========================================
- Coverage     99.0%   99.0%   -0.0%     
=========================================
  Files          225     230      +5     
  Lines        16424   16442     +18     
  Branches      1792    1792             
=========================================
+ Hits         16253   16265     +12     
- Misses         171     177      +6     

Files	Coverage Δ
include/boost/decimal/decimal128.hpp	98.0% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal128_fast.hpp	96.6% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal32.hpp	99.8% <100.0%> (ø)
include/boost/decimal/decimal32_fast.hpp	97.3% <100.0%> (ø)
include/boost/decimal/decimal64.hpp	99.3% <100.0%> (ø)
include/boost/decimal/decimal64_fast.hpp	98.7% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/detail/div_impl.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/emulated128.hpp	97.4% <100.0%> (-0.2%)	⬇️
include/boost/decimal/detail/mul_impl.hpp	92.9% <100.0%> (-0.4%)	⬇️
include/boost/decimal/detail/sub_impl.hpp	97.4% <100.0%> (+1.1%)	⬆️

... and 12 files with indirect coverage changes

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