Improve performance of splitmix discard

[mborland]

Closes: #141

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.79%. Comparing base (be37499) to head (852cb0e).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #142      +/-   ##
===========================================
- Coverage    95.85%   95.79%   -0.06%     
===========================================
  Files          115      115              
  Lines         8128     7642     -486     
===========================================
- Hits          7791     7321     -470     
+ Misses         337      321      -16     

Files with missing lines	Coverage Δ
include/boost/random/splitmix64.hpp	100.00% <100.00%> (ø)

... and 92 files with indirect coverage changes

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