src: use std::list for at_exit_functions

This change was suggested by bnoordhuis in the following comment:
https://github.com/nodejs/node/pull/9163#discussion_r84264628

Not including any tests as this is covered by test/addons/at-exit.

PR-URL: https://github.com/nodejs/node/pull/12255


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>