path: improve POSIX path.join() performance

Performance gains are ~4x (~1.5us), but still much slower than a naive
approach. There is some duplicate work done between join(), normalize()
and normalizeArray() so additional optimizations are possible.

Note that this only improves the POSIX implementation.

Thanks to @isaacs and @othiym23 for helping with this optimization.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>