Improve the performance of our rounding step

[mborland]

Use grandlund Montgomery rather than divmod since the former is far faster with our larger, synthetic types

[codecov]

Codecov Report

❌ Patch coverage is 97.90210% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.4%. Comparing base (697dbc7) to head (6899481).

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/fenv_rounding.hpp	96.7%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1390     +/-   ##
=========================================
- Coverage     98.5%   98.4%   -0.0%     
=========================================
  Files          288     288             
  Lines        22831   22972    +141     
  Branches      1985    1997     +12     
=========================================
+ Hits         22473   22604    +131     
- Misses         358     368     +10     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal_fast128_t.hpp	96.0% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal_fast32_t.hpp	97.3% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal_fast64_t.hpp	97.5% <100.0%> (+0.1%)	⬆️
...e/boost/decimal/detail/cmath/impl/sqrt128_impl.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/u256.hpp	82.5% <100.0%> (+1.6%)	⬆️
include/boost/decimal/detail/fenv_rounding.hpp	97.0% <96.7%> (-0.8%)	⬇️

... and 1 file with indirect coverage changes

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