Reduce code duplication in 32 bit types

[mborland]

Adds a method for one shot decoding and completely removes the subtraction implementation as we can just use a - b = a + (-b)

[mborland]

Uses techniques from #857 to speed things way up.

Current develop:

===== Addition =====
Addition<decimal32  >: 1762595    us (s=999967341040)
Addition<dec32_fast >: 889756     us (s=999967341040)

===== Subtraction =====
Subtraction<decimal32  >: 1490533    us (s=166646259125)
Subtraction<dec32_fast >: 897638     us (s=166646259125)

with this PR:

===== Addition =====
Addition<decimal32  >: 613029     us (s=999967341040)
Addition<dec32_fast >: 628661     us (s=999967341040)

===== Subtraction =====
Subtraction<decimal32  >: 780128     us (s=166646259125)
Subtraction<dec32_fast >: 682079     us (s=166646259125)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (da32132) to head (19f9e2f).
Report is 29 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #858     +/-   ##
=========================================
+ Coverage     98.8%   98.9%   +0.1%     
=========================================
  Files          232     233      +1     
  Lines        16827   16824      -3     
  Branches      1813    1817      +4     
=========================================
  Hits         16625   16625             
+ Misses         202     199      -3     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal32.hpp	98.6% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal32_fast.hpp	96.5% <100.0%> (+0.4%)	⬆️
include/boost/decimal/detail/add_impl.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/fma.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/components.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/fenv_rounding.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/sub_impl.hpp	95.5% <ø> (-1.5%)	⬇️
test/random_decimal32_fast_comp.cpp	100.0% <ø> (ø)

... and 4 files with indirect coverage changes

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