Fix slowdown with 64 bit mul

[mborland]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (d625b19) to head (b71d1c9).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #873     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          233     233             
  Lines        16779   16784      +5     
  Branches      1799    1800      +1     
=========================================
+ Hits         16579   16583      +4     
- Misses         200     201      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal64.hpp	99.1% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64_fast.hpp	98.0% <ø> (ø)
include/boost/decimal/detail/mul_impl.hpp	95.8% <100.0%> (-0.1%)	⬇️
test/random_decimal64_math.cpp	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

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