Improve comparison operators for decimal32 and decimal32_fast

[mborland]

Closes: #702

New runtimes in the comp benchmarks are 0.776 for decimal32 and 0.809 for decimal32_fast.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (1ff2bbd) to head (ae22722).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #703     +/-   ##
=========================================
- Coverage     99.2%   99.2%   -0.0%     
=========================================
  Files          230     230             
  Lines        16290   16309     +19     
  Branches      1757    1767     +10     
=========================================
+ Hits         16146   16164     +18     
- Misses         144     145      +1     

Files	Coverage Δ
include/boost/decimal/decimal32.hpp	99.8% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal32_fast.hpp	97.1% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/comparison.hpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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