Add CUDA support and testing for mixed decimal arithmetic

[mborland]

Closes: #1373

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.6%. Comparing base (73bf06e) to head (8120ce8).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1376     +/-   ##
=========================================
- Coverage     98.6%   98.6%   -0.0%     
=========================================
  Files          284     284             
  Lines        22676   22675      -1     
  Branches      1974    1974             
=========================================
- Hits         22343   22338      -5     
- Misses         333     337      +4     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	96.0% <100.0%> (ø)
include/boost/decimal/decimal32_t.hpp	95.9% <100.0%> (ø)
include/boost/decimal/decimal64_t.hpp	94.8% <100.0%> (ø)
include/boost/decimal/decimal_fast32_t.hpp	97.8% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/detail/comparison.hpp	96.8% <100.0%> (ø)
.../boost/decimal/detail/mixed_decimal_arithmetic.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/to_decimal.hpp	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

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