Add 128-bit DPD conversions

[mborland]

Closes: #741
Closes: #641

[codecov]

Codecov Report

Attention: Patch coverage is 88.05970% with 8 lines in your changes missing coverage. Please review.

Project coverage is 99.1%. Comparing base (16712c0) to head (d55f599).
Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/dpd_conversion.hpp	87.4%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #743     +/-   ##
=========================================
- Coverage     99.2%   99.1%   -0.0%     
=========================================
  Files          226     226             
  Lines        16683   16751     +68     
  Branches      1802    1809      +7     
=========================================
+ Hits         16537   16596     +59     
- Misses         146     155      +9     

Files with missing lines	Coverage Δ
include/boost/decimal/bid_conversion.hpp	100.0% <ø> (ø)
include/boost/decimal/decimal128.hpp	98.1% <ø> (ø)
include/boost/decimal/decimal128_fast.hpp	97.6% <ø> (ø)
test/test_dpd_conversions.cpp	100.0% <100.0%> (ø)
include/boost/decimal/dpd_conversion.hpp	96.6% <87.4%> (-2.0%)	⬇️

... and 2 files with indirect coverage changes

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