lib: remove let from for loops

This is a known de-opt. It may not be 100% necessary in all cases but it
seems like a decent enough idea to avoid it.

PR-URL: https://github.com/nodejs/node/pull/8873


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>