Increase precision of 128-bit division

[mborland]

Also completely removes the now incorrect subtraction impl

Closes: #1041

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.3%. Comparing base (b212b95) to head (d032639).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1065     +/-   ##
=========================================
- Coverage     98.3%   98.3%   -0.0%     
=========================================
  Files          254     253      -1     
  Lines        16622   16593     -29     
  Branches      1850    1851      +1     
=========================================
- Hits         16339   16310     -29     
  Misses         283     283             

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	97.9% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal32_t.hpp	96.6% <ø> (ø)
include/boost/decimal/decimal64_t.hpp	97.2% <ø> (ø)
include/boost/decimal/decimal_fast128_t.hpp	97.9% <100.0%> (ø)
include/boost/decimal/decimal_fast64_t.hpp	97.9% <ø> (ø)
include/boost/decimal/detail/u256.hpp	98.6% <100.0%> (+0.1%)	⬆️

... and 1 file with indirect coverage changes

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