Implement proper 64 bit FMA

[mborland]

Also updates the special function evaluation to use our unchecked FMA to speed things up as suggested by @ckormanyos

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (50b689c) to head (2030636).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #779     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          225     225             
  Lines        16886   16915     +29     
  Branches      1804    1806      +2     
=========================================
+ Hits         16686   16714     +28     
- Misses         200     201      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/cmath/fma.hpp	100.0% <100.0%> (ø)
.../decimal/detail/cmath/impl/remez_series_result.hpp	100.0% <100.0%> (ø)
...decimal/detail/cmath/impl/taylor_series_result.hpp	100.0% <100.0%> (ø)

... and 4 files with indirect coverage changes

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