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.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
src