Add one-shot decoding for 64-bit types and improve mul

[mborland]

Closes: #864
Closes: #824

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (9b9548d) to head (c9e4f55).
Report is 9 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #866     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          233     233             
  Lines        16820   16840     +20     
  Branches      1817    1817             
=========================================
+ Hits         16619   16640     +21     
+ Misses         201     200      -1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal64.hpp	99.1% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64_fast.hpp	98.1% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/detail/components.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/mul_impl.hpp	96.0% <100.0%> (+0.5%)	⬆️

... and 1 file with indirect coverage changes

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