Add conversions for builtin 128-bit types

[mborland]

Closes: #337

[codecov]

Codecov Report

Merging #340 (549668f) into develop (270204a) will increase coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #340     +/-   ##
=========================================
+ Coverage     97.6%   97.6%   +0.1%     
=========================================
  Files           73      73             
  Lines         3342    3346      +4     
=========================================
+ Hits          3261    3265      +4     
  Misses          81      81             

Files	Coverage Δ
include/boost/decimal/decimal128.hpp	95.6% <ø> (ø)
include/boost/decimal/decimal32.hpp	99.9% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64.hpp	98.6% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/emulated128.hpp	91.5% <100.0%> (ø)

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