fs: replace regexp with function

Replacing the path separator-finding regexp with a custom function
results in a measurable improvement in performance.

PR-URL: https://github.com/nodejs/node/pull/10789


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>