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be the same as ↔.

This is a fairly intrusive change, but at least makes notations more
consistent, and often shorter because fewer parentheses are needed. Note
that viewshifts already had the same precedence as →.

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

We git this from monotonicity now.

- Make the carrier argument of the constructors for the canonical structures
  cofeT and cmraT explicit. This way we make sure the carrier is properly
  exposed, instead of some alias of the carrier.
- Make derived constructions (such as discreteC and discreteR) notations
  instead of definitions. This is yet again to make sure that the carrier is
  properly exposed.
- Turn DRA into a canonical structure (it used to be a type class).

This fixes some issues, notably it fixes some broken rewrites in algebra/sts
and it makes canonical structures work properly with dec_agree.

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.

Initial commit by Amin Timany.