This reverts merge request !309

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.

This existed at least as far back as
361308c7, 8 years ago.

- Define set-like notation `{[+ x1; ..; xn ]}` for multisets in terms of the new
  singleton class and disjoint union `⊎`.
- Remove `SemiSet` instance for multisets.
- Prove lemmas regarding `∈` and `∉` for multisets since we no longer get the
  generic versions for sets.
- Provide `SetUnfoldElemOf` instances for multisets since we no longer get the
  generic versions for sets.
- Prove lemmas new regarding `∈` and `∉` for `∩`

Fixes #100, #98 and #87.

This MR is an alternative to !232.

Based on
https://github.com/coq/coq/issues/9058#issuecomment-496479506.

Fixes new Coq master warning deprecated-hint-without-locality.

These were very specific, there were no lemmas about them. They were used
back in the days for some specific things in my C semantics.

Fix in preparation for https://github.com/coq/coq/pull/13188