Reduce language requirement for constexpr BID conversions

[mborland]

Closes: #737

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (6e237c6) to head (71fe572).
Report is 14 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #739     +/-   ##
=========================================
- Coverage     99.2%   99.2%   -0.0%     
=========================================
  Files          226     226             
  Lines        16635   16609     -26     
  Branches      1792    1792             
=========================================
- Hits         16491   16464     -27     
- Misses         144     145      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/bid_conversion.hpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal128.hpp	98.1% <ø> (ø)
include/boost/decimal/decimal32.hpp	98.8% <ø> (ø)
include/boost/decimal/decimal64.hpp	99.3% <ø> (ø)
test/test_bid_conversions.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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