Based on an idea and WIP commits of J-H. Jourdan: the core of a CMRA
A is now a partial function A → option A.

TODO: define sum CMRA
TODO: remove one shot CMRA and define it in terms of sum

Rationale: to make the code closer to what is on paper, I want the notations
to look like quantifiers, i.e. have a binder built-in. I thus introduced the
following notations:

  [★ map] k ↦ x ∈ m, P
  [★ set] x ∈ X, P

The good thing - contrary to the notations that we had before that required an
explicit lambda - is that type annotations of k and x are now not printed
making goals much easier to read.

This gets rid of the (ambiguous) notation %l, because we can declare
LitLoc as a coercion. It also shortens the code.

It is not a library; it does not contain code, but instead is a core
part of heap_lang.

Also remove some superfluous map_ prefixes.

Thanks to Amin Timany for the suggestion.

Add both non-expansive and contractive functors, and bundle them for the general Iris instance as well as the global functor construction

This allows us to move the \later in the user-defined functor to any place we want.
In particular, we can now have "\later (iProp -> iProp)" in the ghost CMRA.

make the global functor stuff in the various constructions more uniform; change it such that barrier/proof does not have to repeat the functors it needs

This cleans up some ad-hoc stuff and prepares for a generalization
of saved propositions.

The changes are probably necessary because rewrite now tries harder not to
instantiate evars, which it always said it would not do.

It now turns setoid equalities into Leibniz equalities when possible,
and substitutes those.

* Use sig instead of sigT: the proof is a Prop after all
* Tweak implicit arguments
* Shorten proof of sigma

This way it behaves better for discrete CMRAs.