promises: more robust stringification

PR-URL: https://github.com/nodejs/node/pull/13784
Fixes: https://github.com/nodejs/node/issues/13771


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>