Improved comparisons for decimal64

[mborland]

Provides a >30% speedup to decimal64s comparisons timing

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (9543df5) to head (13726f3).
Report is 15 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #730     +/-   ##
=========================================
- Coverage     99.2%   99.2%   -0.0%     
=========================================
  Files          224     224             
  Lines        16398   16397      -1     
  Branches      1781    1780      -1     
=========================================
- Hits         16255   16254      -1     
  Misses         143     143             

Files with missing lines	Coverage Δ
include/boost/decimal/decimal64.hpp	99.3% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/detail/comparison.hpp	99.4% <100.0%> (+0.1%)	⬆️

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