Fix loss of precision in operator% and remove all individual (and duplicated) impls

[mborland]

Closes: #1314

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.9%. Comparing base (a8abd00) to head (d3ecf65).
⚠️ Report is 17 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/mod_impl.hpp	82.4%	3 Missing ⚠️
include/boost/decimal/decimal_fast128_t.hpp	0.0%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1317     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          275     276      +1     
  Lines        17886   17902     +16     
  Branches      1908    1909      +1     
=========================================
+ Hits         17688   17704     +16     
  Misses         198     198             

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	98.2% <100.0%> (+1.4%)	⬆️
include/boost/decimal/decimal32_t.hpp	96.5% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal64_t.hpp	95.3% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal_fast32_t.hpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal_fast64_t.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/power_tables.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/u256.hpp	96.4% <ø> (-3.6%)	⬇️
test/random_decimal128_math.cpp	100.0% <100.0%> (ø)
test/random_decimal32_math.cpp	100.0% <100.0%> (ø)
test/random_decimal64_math.cpp	100.0% <100.0%> (ø)
... and 2 more

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