Obey current rounding mode with rint/lrint/nearbyint

[mborland]

Closes: #1112

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.4%. Comparing base (7d44a84) to head (a9f3040).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1128     +/-   ##
=========================================
+ Coverage     99.4%   99.4%   +0.1%     
=========================================
  Files          246     247      +1     
  Lines        16808   16831     +23     
  Branches      1814    1813      -1     
=========================================
+ Hits         16701   16726     +25     
+ Misses         107     105      -2     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/cmath/rint.hpp	95.0% <100.0%> (-0.1%)	⬇️
test/github_issue_1112.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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