Refactor decimal32 FMA implementation

[mborland]

Closes: #554

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.8%. Comparing base (36c8560) to head (df3872e).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #555     +/-   ##
=========================================
+ Coverage     98.8%   98.8%   +0.1%     
=========================================
  Files          192     192             
  Lines        11511   11514      +3     
  Branches      1386    1391      +5     
=========================================
+ Hits         11372   11375      +3     
  Misses         139     139             

Files	Coverage Δ
include/boost/decimal/decimal32.hpp	99.7% <ø> (-<0.1%)	⬇️
include/boost/decimal/detail/cmath/fma.hpp	100.0% <100.0%> (ø)
test/test_cmath.cpp	100.0% <ø> (ø)

... and 1 file with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 36c8560...df3872e. Read the comment docs.