Use `_True` and `_False` for the specific cases.

Weaken premise of `map_filter_delete_not` and make it consistent with `map_filter_insert_not'`.

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

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

This also applies to `(un)curry{3,4}`, `gmap_(un)curry`, and `h(un)curry`.

This fixes issue #76.

The code includes a horrible hack that should removed once support for Coq versions prior
to 8.13 is dropped.