buffer: backport allocUnsafeSlow

This backports the new `Buffer.allocUnsafeSlow()` API for v5.

This backport includes the new API, test cases, and docs additions.
Already present API and testcases were not changed.

PR-URL: https://github.com/nodejs/node/pull/7169


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>