Add CUDA support to decimal128_t

[mborland]

Closes: #1356

[mborland]

These types require an 8k stack to be used. That's pretty big and I'm not sure how valuable this will be.

[codecov]

Codecov Report

❌ Patch coverage is 99.20635% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.6%. Comparing base (a07d93a) to head (6c75014).
⚠️ Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/u256.hpp	97.3%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1360     +/-   ##
=========================================
- Coverage     98.6%   98.6%   -0.0%     
=========================================
  Files          283     283             
  Lines        22625   22623      -2     
  Branches      1972    1972             
=========================================
- Hits         22292   22289      -3     
- Misses         333     334      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	96.0% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/add_impl.hpp	97.2% <100.0%> (ø)
include/boost/decimal/detail/div_impl.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/i256.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/mul_impl.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/u256.hpp	80.9% <97.3%> (ø)

... and 2 files with indirect coverage changes

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