Write them all using `↔️` and consistently use the `_iff` suffix.

some lemmas about list submseteq

See merge request iris/stdpp!305

Add lemma `map_singleton_subseteq_l` and `map_singleton_subseteq`.

See merge request iris/stdpp!308

Make singleton/`⊆` lemmas consistent with Iris's singleton/`≼`

See merge request iris/stdpp!306

Rename `gmultiset_elem_of_singleton_subseteq` → `gmultiset_singleton_subseteq_l` and swap order to be consistent with Iris's `singleton_included_l`.

Add lemmas that say that `curry{3,4}?` and `uncurry{3,4}?` are inverses.

See merge request iris/stdpp!304

solve_ndisj: handle goals containing _ ∖ _ ∖ _

See merge request iris/stdpp!303

`Params` and `Proper` instances for `curry` and friends

See merge request iris/stdpp!302

solve_ndisj: solve more goals involving chains of differences

See merge request iris/stdpp!300