Remove coercion `iMsg_car` and `Proper` instances to avoid one accidentally breaking the `iMsg` abstraction.

- Protocols are no longer contractive in the message
- New type `iMsg` for messages to avoid telescopes in protocols
- Better rules for subprotocols that do not involve telescopes, but allow introduction
  and elimination of quantifiers and the payload
- Better notations for protocols
- Notation ⊑ for subprotocols
- Make ⊑ except-0 so one can strip laters when proving a ⊑
- Restore recursive domain equation to push later inwards to support protocols
  that are not contractive in the mssage.
- Proofmode support for easy manipulation of ⊑

Kinded subtyping, better file structure, more setoid stuff, reorganize imports.

By moving the later further to the outside, we can kill many occurences
of `Next`, and get rid of laters/except_0s at awkward positions. Also we
can have a sensible eliminator `proto_elim`.

Moved Iris specific stuff into a utility file.

TODO: fix in-place merge function.

- Allow for binders in protocols.
- Model protocols in CPS style using the COFE solver.
- Change the channel methods so they do not mention sides.
- Lots of refactoring.
- Generalize the `list_sort` example.
- Move stuff to stdpp/Iris.

- Put code and proofs of examples in a single file.
- Rename encode/decode to avoid conflicts with encode/decode (of the countable class) in stdpp.
- Remove useless imports.