Added LTyCopy typeclasses for products and sums
Removed mutable ref to weak ref subtyping as a consequence.

Moved Iris specific stuff into a utility file.

TODO: fix in-place merge function.

Also weaken some specs by adding laters.

- 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.

Maintaining the same example twice is just too much work.

- 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.