is_derive_mulmx

[yosakaon]

Motivation for this change

This PR completes #1891 (comment) with is_derive for matrix multiplication.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

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[yosakaon]

it is worth noting that Hint Extern 0 (is_derive _ _ (fun x => _ *m _) _) => apply: is_derive_mulmx : typeclass_instances. will help solve harder goals while leaving simple apply fails!