Use 256 bit addition and subtraction for 128-bit decimals to avoid loss of precision

[mborland]

Closes: #1287

[ckormanyos]

Nice one and nice find, Matt. It's amazing how some of the function tests and awesome client tests really find edge cases

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.9%. Comparing base (88a0966) to head (653aa3c).
⚠️ Report is 48 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1288     +/-   ##
=========================================
- Coverage     99.0%   98.9%   -0.0%     
=========================================
  Files          269     270      +1     
  Lines        17856   17956    +100     
  Branches      1985    1998     +13     
=========================================
+ Hits         17661   17742     +81     
- Misses         195     214     +19     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	96.8% <100.0%> (-0.1%)	⬇️
include/boost/decimal/decimal_fast128_t.hpp	95.2% <100.0%> (-0.2%)	⬇️
include/boost/decimal/detail/add_impl.hpp	85.6% <100.0%> (-12.5%)	⬇️
include/boost/decimal/detail/fenv_rounding.hpp	96.7% <100.0%> (+0.4%)	⬆️
include/boost/decimal/detail/i256.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/normalize.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/power_tables.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/u256.hpp	100.0% <100.0%> (+0.8%)	⬆️
test/github_issue_1260.cpp	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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