Add cmath support to decimal128_fast

[mborland]

Closes: #637

[codecov]

Codecov Report

Attention: Patch coverage is 94.28571% with 4 lines in your changes missing coverage. Please review.

Project coverage is 98.7%. Comparing base (fc1f412) to head (7172da4).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #638     +/-   ##
=========================================
- Coverage     98.8%   98.7%   -0.1%     
=========================================
  Files          219     219             
  Lines        15657   15717     +60     
  Branches      1718    1720      +2     
=========================================
+ Hits         15459   15497     +38     
- Misses         198     220     +22     

Files	Coverage Δ
include/boost/decimal/decimal128_fast.hpp	97.7% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/cmath/fma.hpp	100.0% <100.0%> (ø)
...lude/boost/decimal/detail/cmath/impl/asin_impl.hpp	100.0% <ø> (ø)
...ecimal/detail/cmath/impl/assoc_legendre_lookup.hpp	75.0% <ø> (ø)
...lude/boost/decimal/detail/cmath/impl/atan_impl.hpp	100.0% <ø> (ø)
...clude/boost/decimal/detail/cmath/impl/cos_impl.hpp	100.0% <100.0%> (ø)
...lude/boost/decimal/detail/cmath/impl/cosh_impl.hpp	100.0% <ø> (ø)
...ude/boost/decimal/detail/cmath/impl/expm1_impl.hpp	100.0% <ø> (ø)
...de/boost/decimal/detail/cmath/impl/lgamma_impl.hpp	100.0% <ø> (ø)
...ude/boost/decimal/detail/cmath/impl/log1p_impl.hpp	100.0% <ø> (ø)
... and 14 more

... and 3 files with indirect coverage changes

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