Add modules support

[mborland]

Export a module called boost2.decimal for consumption

Closes: #268

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.8%. Comparing base (480249d) to head (3700578).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #484     +/-   ##
=========================================
+ Coverage     98.8%   98.8%   +0.1%     
=========================================
  Files          176     176             
  Lines        11279   11281      +2     
=========================================
+ Hits         11134   11136      +2     
  Misses         145     145             

Files	Coverage Δ
include/boost/decimal/charconv.hpp	95.2% <ø> (ø)
include/boost/decimal/cmath.hpp	100.0% <100.0%> (ø)
include/boost/decimal/cstdlib.hpp	98.3% <ø> (ø)
include/boost/decimal/decimal128.hpp	97.4% <ø> (ø)
include/boost/decimal/decimal32.hpp	99.7% <ø> (ø)
include/boost/decimal/decimal64.hpp	98.9% <ø> (ø)
include/boost/decimal/detail/apply_sign.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/bit_cast.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/buffer_sizing.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/check_non_finite.hpp	100.0% <ø> (ø)
... and 91 more

... and 2 files with indirect coverage changes

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