Implement basic arithmetic for decimal128_fast

[mborland]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 98.88889% with 3 lines in your changes missing coverage. Please review.

Project coverage is 99.1%. Comparing base (026f3fa) to head (8e15a95).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #635     +/-   ##
=========================================
+ Coverage     99.1%   99.1%   +0.1%     
=========================================
  Files          217     218      +1     
  Lines        15014   15212    +198     
  Branches      1648    1668     +20     
=========================================
+ Hits         14866   15064    +198     
  Misses         148     148             

Files	Coverage Δ
include/boost/decimal/decimal128.hpp	97.5% <100.0%> (-0.1%)	⬇️
include/boost/decimal/detail/add_impl.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/fma.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/div_impl.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/mul_impl.hpp	100.0% <100.0%> (ø)
test/random_decimal128_fast_math.cpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal128_fast.hpp	96.8% <99.0%> (+9.8%)	⬆️
include/boost/decimal/detail/sub_impl.hpp	96.6% <92.9%> (-1.7%)	⬇️

... and 2 files with indirect coverage changes

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