[PATCH] Integration of parallel standard algorithms for c++17

[Thomas Rodgers]

This patch has been held up by licensing issues, specifically, awaiting
upstream's plans to relicense the code from Apache 2.0 to the libc++ license -

https://llvm.org/docs/DeveloperPolicy.html#license

We expect the relicensing to conclude within the next few weeks. At that
time I will be resubmiting this patch with the new license.

-- Tom