PPQSort is supposedly quite fast: https://github.com/GabTux/PPQSort
More importantly, it does not depend on TBB fuckery, so there's no longer
a need to link with an external library that people may or may not have.
The NO_PARALLEL_STL option is out, as it was provided solely to deal with
TBB being not available. Sequential sorting is still used on emscripten.
