buffer,util: refactor for performance

internal/util.js definied toInteger() and toLength() but they were only
used by buffer.js. Inlining these small functions results in a small but
statistically-significant performance gain.

PR-URL: https://github.com/nodejs/node/pull/12153


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>