Improve comparisons for decimal32

[mborland]

Breaks the decoding of the type into multiple steps rather than doing it all up front, because it many cases that is unnecessary. Results in about a 6.5% speedup in all operations across decimal32.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (0478b0c) to head (7f63f4f).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #729     +/-   ##
=========================================
- Coverage     99.2%   99.2%   -0.0%     
=========================================
  Files          224     224             
  Lines        16375   16398     +23     
  Branches      1778    1781      +3     
=========================================
+ Hits         16233   16255     +22     
- Misses         142     143      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal32.hpp	98.8% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal64.hpp	99.3% <ø> (ø)
include/boost/decimal/detail/comparison.hpp	99.4% <100.0%> (+0.2%)	⬆️

... and 2 files with indirect coverage changes

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