Add data() member to float128

[mborland]

Closes: #743

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.2%. Comparing base (d6de341) to head (9bd84a2).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #761   +/-   ##
=======================================
  Coverage     96.2%   96.2%           
=======================================
  Files          301     301           
  Lines        29462   29462           
=======================================
  Hits         28328   28328           
  Misses        1134    1134           

Files with missing lines	Coverage Δ
include/boost/multiprecision/float128.hpp	84.1% <ø> (ø)

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