Many improvements to `multiset_solver`

See merge request !231

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.

Remove singleton notations for tuples

See merge request iris/stdpp!233

coPset: some lemmas about infinity

See merge request !230

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

clarify Pmap_raw comment

See merge request !229

Add several simple lemmas (mostly about list and map filter).

See merge request !226

Avoid relying on buggy simpl never behavior

See merge request iris/stdpp!228

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.

add Nat_iter_mul

See merge request !227

add pred_infinite_surj

See merge request iris/stdpp!225

rename elem_of_equiv_L -> set_eq, and set_equiv_spec_L -> set_eq_subseteq

See merge request iris/stdpp!210

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