Turn `x ∈ X` only into `0 < multiplicity x X` at leaves of `∈` to enable better first-order reasoning.

This ensures they are always picked before those of sets.

Proofs by Joshua Yanowski

This reverts commit ec5b6bd8.

The FIXME seems to not just rely on https://github.com/coq/coq/issues/5735
since it fails with Coq 8.10 and 8.11

Alternate take on
https://gitlab.inria.fr/bertot/stdpp/-/commit/895121f919c6f1332f41f658ce7f850e391eb49e,
which is used as an overlay in Coq for
https://github.com/coq/coq/pull/13448.

rename elem_of_equiv -> set_equiv and set_equiv_spec -> set_equiv_subseteq, and rename some instances to get out of the way

names for big operators in Iris.