Improve 128 bit comparisons

[mborland]

Closes: #701
Closes: #707
Closes: #708

New benchmarks for decimal128 show >20% speedup and decimal128_fast nets ~30%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.1%. Comparing base (80872c3) to head (2b0cce3).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #709     +/-   ##
=========================================
- Coverage     99.2%   99.1%   -0.0%     
=========================================
  Files          230     230             
  Lines        16337   16339      +2     
  Branches      1765    1766      +1     
=========================================
- Hits         16191   16190      -1     
- Misses         146     149      +3     

Files	Coverage Δ
include/boost/decimal/decimal128.hpp	98.1% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal128_fast.hpp	97.6% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/comparison.hpp	100.0% <ø> (ø)

... and 6 files with indirect coverage changes

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