Refactor rescale

[mborland]

Closes: #761

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.7%. Comparing base (defb2dc) to head (384155e).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #762     +/-   ##
=========================================
- Coverage     99.1%   98.7%   -0.3%     
=========================================
  Files          227     225      -2     
  Lines        17372   16811    -561     
  Branches      1814    1797     -17     
=========================================
- Hits         17205   16587    -618     
- Misses         167     224     +57     

Files with missing lines	Coverage Δ
include/boost/decimal/cmath.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/rescale.hpp	94.8% <100.0%> (ø)
test/random_decimal128_fast_math.cpp	100.0% <ø> (ø)
test/random_decimal32_fast_math.cpp	100.0% <ø> (ø)
test/random_decimal64_fast_math.cpp	100.0% <ø> (ø)
test/random_decimal64_math.cpp	100.0% <ø> (ø)
test/test_fixed_width_trunc.cpp	100.0% <100.0%> (ø)

... and 64 files with indirect coverage changes

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