Fix detection of pure powers of 10 for large significands

[mborland]

Closes: #1110

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.4%. Comparing base (66b697f) to head (26b474e).
⚠️ Report is 14 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1130     +/-   ##
=========================================
+ Coverage     99.4%   99.4%   +0.1%     
=========================================
  Files          247     248      +1     
  Lines        16831   16855     +24     
  Branches      1813    1813             
=========================================
+ Hits         16724   16748     +24     
  Misses         107     107             

Files with missing lines	Coverage Δ
include/boost/decimal/detail/cmath/cbrt.hpp	96.7% <100.0%> (ø)
include/boost/decimal/detail/cmath/log10.hpp	94.5% <100.0%> (ø)
include/boost/decimal/detail/cmath/sqrt.hpp	97.6% <100.0%> (ø)
include/boost/decimal/detail/power_tables.hpp	100.0% <ø> (ø)
test/github_issue_1110.cpp	100.0% <100.0%> (ø)
test/test_remove_trailing_zeros.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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