Faster dec128 mul

[mborland]

Checking the number of digits is a O(log(n)) operation. If we can fit the result of significand multiplication into the range of a u128 then we avoid the 256-bit multiplication all together.

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.2%. Comparing base (cb7f7b0) to head (c4bd2af).
Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/mul_impl.hpp	93.8%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #733     +/-   ##
=========================================
+ Coverage     99.2%   99.2%   +0.1%     
=========================================
  Files          224     224             
  Lines        16395   16401      +6     
  Branches      1779    1780      +1     
=========================================
+ Hits         16252   16261      +9     
+ Misses         143     140      -3     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128.hpp	98.1% <ø> (-<0.1%)	⬇️
include/boost/decimal/decimal128_fast.hpp	97.6% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/mul_impl.hpp	94.2% <93.8%> (+1.3%)	⬆️

... and 2 files with indirect coverage changes

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