Make uint128 public since we return in a few places to the user

[mborland]

Closes: #817

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (437d912) to head (f7880fe).
Report is 9 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #822     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          230     230             
  Lines        17055   17056      +1     
  Branches      1808    1808             
=========================================
  Hits         16864   16864             
- Misses         191     192      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/bid_conversion.hpp	100.0% <100.0%> (ø)
test/test_big_uints.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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