This requirement was useful in Iris 2.0: in order to ensure that ownership of
the physical state was timeless, we required the ghost CMRA to have a timeless
unit. To avoid having additional type class parameters, or having to extend the
algebraic hierarchy, we required the units of any CMRA to be timeless.

In Iris 3.0, this issue no longer applies: ownership of the physical state is
ghost ownership in the global CMRA, whose unit is always timeless.

Thanks to Jeehoon Kang for spotting this unnecessary requirement.

This is more consistent with the definition of the extension order, which
is also defined in terms of an existential.

This is inspired by ssr, and makes unification faster if it goes
right-to-left.

See https://sympa.inria.fr/sympa/arc/ssreflect/2013-11/msg00010.html

This way, we can use eapply instead of class_apply, which is used
when the instances are defined using the Instance command. It seems
that eapply is stronger as class_apply, and as such solves some issues
when canonical structures have type class parameters, for example:

  Goal Op (option (dec_agree nat)). apply _.

This failed, but is fixed by this commit.

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

- 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.