Improve u128 x u128

[mborland]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (9543df5) to head (ad2b7cf).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #731     +/-   ##
=========================================
- Coverage     99.2%   99.2%   -0.0%     
=========================================
  Files          224     224             
  Lines        16398   16395      -3     
  Branches      1781    1779      -2     
=========================================
- Hits         16255   16252      -3     
  Misses         143     143             

Files with missing lines	Coverage Δ
include/boost/decimal/detail/emulated256.hpp	100.0% <100.0%> (ø)

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