Improve num_digits() for uint256_t

[mborland]

Improves decimal128 by 6.369x and decimal128_fast by 4.651 in >=C++17 modes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.5%. Comparing base (556a5d2) to head (0424598).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #660     +/-   ##
=========================================
+ Coverage     98.5%   98.5%   +0.1%     
=========================================
  Files          221     221             
  Lines        15661   15673     +12     
  Branches      1722    1726      +4     
=========================================
+ Hits         15416   15428     +12     
  Misses         245     245             

Files	Coverage Δ
...lude/boost/decimal/detail/integer_search_trees.hpp	100.0% <ø> (ø)
test/test_big_uints.cpp	100.0% <100.0%> (ø)

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