Allow separate header usage and test all includes

[mborland]

Closes: #804

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (1feeee0) to head (716362b).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #818     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          230     230             
  Lines        17067   17055     -12     
  Branches      1807    1808      +1     
=========================================
- Hits         16876   16864     -12     
  Misses         191     191             

Files with missing lines	Coverage Δ
include/boost/decimal/bid_conversion.hpp	100.0% <ø> (ø)
include/boost/decimal/charconv.hpp	88.4% <100.0%> (-0.6%)	⬇️
include/boost/decimal/cstdio.hpp	95.6% <ø> (ø)
include/boost/decimal/cstdlib.hpp	98.0% <ø> (ø)
include/boost/decimal/detail/io.hpp	100.0% <ø> (ø)
include/boost/decimal/dpd_conversion.hpp	96.6% <ø> (ø)
include/boost/decimal/hash.hpp	100.0% <ø> (ø)
include/boost/decimal/literals.hpp	100.0% <ø> (ø)
include/boost/decimal/string.hpp	100.0% <ø> (ø)
test/random_decimal32_comp.cpp	100.0% <ø> (ø)
... and 15 more

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