Improve some basic 128-bit operations

[mborland]

Closes: #557

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.8%. Comparing base (701d32e) to head (576011b).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #560     +/-   ##
=========================================
- Coverage     98.8%   98.8%   -0.0%     
=========================================
  Files          192     192             
  Lines        11514   11480     -34     
  Branches      1391    1389      -2     
=========================================
- Hits         11375   11341     -34     
  Misses         139     139             

Files	Coverage Δ
include/boost/decimal/decimal128.hpp	97.2% <100.0%> (-<0.1%)	⬇️
...lude/boost/decimal/detail/integer_search_trees.hpp	100.0% <100.0%> (ø)
test/test_to_chars.cpp	100.0% <ø> (ø)

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