Better 64 bit comparisons

[mborland]

Closes: #704

Knocks decimal64 down to 0.825 and ~parity for decimal64_fast

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (c115908) to head (0d04df2).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #705     +/-   ##
=========================================
- Coverage     99.2%   99.2%   -0.0%     
=========================================
  Files          230     230             
  Lines        16309   16337     +28     
  Branches      1767    1765      -2     
=========================================
+ Hits         16164   16191     +27     
- Misses         145     146      +1     

Files	Coverage Δ
include/boost/decimal/charconv.hpp	95.3% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal32.hpp	99.8% <ø> (ø)
include/boost/decimal/decimal32_fast.hpp	96.7% <100.0%> (-0.3%)	⬇️
include/boost/decimal/decimal64.hpp	99.3% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal64_fast.hpp	98.6% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/comparison.hpp	100.0% <100.0%> (ø)
test/test_cmath.cpp	100.0% <100.0%> (ø)
test/test_to_chars.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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