Faster dec128 fast

[mborland]

Closes: #656

Provides 2.375x speedup vs develop and 3.236x speedup vs decimal128

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.5%. Comparing base (c7aed9d) to head (3c87b81).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #658     +/-   ##
=========================================
- Coverage     98.5%   98.5%   -0.0%     
=========================================
  Files          221     221             
  Lines        15696   15661     -35     
  Branches      1723    1722      -1     
=========================================
- Hits         15452   15416     -36     
- Misses         244     245      +1     

Files	Coverage Δ
include/boost/decimal/decimal128_fast.hpp	97.0% <100.0%> (-0.7%)	⬇️
include/boost/decimal/detail/add_impl.hpp	100.0% <100.0%> (ø)

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