Allow implicit widening between decimal types and keep explicit narrowing

[mborland]

Closes: #806

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (b4f8cad) to head (a0ea406).
Report is 10 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #816     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          229     230      +1     
  Lines        17004   17067     +63     
  Branches      1809    1807      -2     
=========================================
+ Hits         16814   16876     +62     
- Misses         190     191      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128.hpp	97.8% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal32.hpp	98.6% <ø> (ø)
include/boost/decimal/decimal32_fast.hpp	96.5% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64.hpp	98.8% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64_fast.hpp	98.2% <100.0%> (+0.1%)	⬆️
test/test_legal_implicit_conversions.cpp	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

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