Improve dec64 multiplication

[mborland]

Closes: #653

Provides an ~40% speedup to decimal64 by replacing O(log(N)) comparisions with 1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.5%. Comparing base (1cd6c70) to head (e57bff0).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #655     +/-   ##
=========================================
- Coverage     98.5%   98.5%   -0.0%     
=========================================
  Files          221     221             
  Lines        15692   15696      +4     
  Branches      1724    1723      -1     
=========================================
- Hits         15455   15452      -3     
- Misses         237     244      +7     

Files	Coverage Δ
include/boost/decimal/detail/emulated256.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/mul_impl.hpp	96.7% <100.0%> (-0.1%)	⬇️
include/boost/decimal/detail/power_tables.hpp	100.0% <ø> (ø)

... and 2 files with indirect coverage changes

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