Add Decimal64 fast to benchmarks

[mborland]

Closes: #614
Closes: #594

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.0%. Comparing base (a88b965) to head (f5605e7).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #615   +/-   ##
=======================================
  Coverage     99.0%   99.0%           
=======================================
  Files          212     212           
  Lines        14696   14696           
  Branches      1615    1615           
=======================================
  Hits         14548   14548           
  Misses         148     148           

Files	Coverage Δ
include/boost/decimal/detail/to_integral.hpp	100.0% <100.0%> (ø)

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