hint is_derive_mulmx

[yosakaon]

Motivation for this change

Fixes #2069.
It seems like the best course of action to me to be able to use typeclass inference, and syntax follows other hints about is_derive in the file, but notice #2069 (comment) (eapply works as well). apply: is also used in other hints so I guess it's ok.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

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