util: use template strings

This commit changes string manipulation in favor of template
literals in the `util` module.

PR-URL: https://github.com/nodejs/node/pull/9120


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>