Use the complete range of int128 when performing 128-bit addition and subtraction

[mborland]

Closes: #1260

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.0%. Comparing base (b5789e1) to head (3dfe776).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1275     +/-   ##
=========================================
+ Coverage     99.0%   99.0%   +0.1%     
=========================================
  Files          267     268      +1     
  Lines        17734   17746     +12     
  Branches      1981    1980      -1     
=========================================
+ Hits         17542   17554     +12     
  Misses         192     192             

Files with missing lines	Coverage Δ
include/boost/decimal/detail/add_impl.hpp	98.1% <100.0%> (ø)
test/github_issue_1260.cpp	100.0% <100.0%> (ø)

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