Now, for example, when having equiv (Some x) (Some y) it will
try to find a Proper whose range is an equiv before hitting the
eq instance. My hack is general enough that it works for Forall2,
dist, and so on, too.

It used to be: (P ={E}=> Q) := (True ⊢ (P → |={E}=> Q))
Now it is: (P ={E}=> Q) := (P ⊢ |={E}=> Q)

It is now able to destruct:
- [own γ (a1 ⋅ a1)] into [own γ a1] and [own γ a2]
- [own γ a] into [own γ a] and [own γ a] if [a] is persistent
- [own γ (a,b)] by proceeding recursively.
- [own γ (Some a)] by preceeding resursively.

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