provide a stdx::variant

Given that c++ is coming down the pipeline, it'd useful to provide a stdx variant for 4.0 and earlier for ease of backports / use.

It seems like the author of libc++'s variant impl has a separate project with a header only implementation:
https://github.com/mpark/variant

This ticket is for the work to pull the 3rd party dependency + do the stdx hookup