The absence of this axiom has two consequences:

- We no longer have `■ (P ∗ Q) ⊢ ■ P ∗ ■ Q` and `□ (P ∗ Q) ⊢ □ P ∗ □ Q`,
  and as a result, separating conjunctions in the unrestricted/persistent
  context cannot be eliminated.
- When having `(P -∗ ⬕ Q) ∗ P`, we do not get `⬕ Q ∗ P`. In the proof
  mode this means when having:

    H1 : P -∗ ⬕ Q
    H2 : P

  We cannot say `iDestruct ("H1" with "H2") as "#H1"` and keep `H2`.

However, there is now a type class `PositiveBI PROP`, and when there is an
instance of this type class, one gets the above reasoning principle back.

TODO: Can we describe positivity of individual propositions instead of the
whole BI? That way, we would get the above reasoning principles even when
the BI is not positive, but the propositions involved are.

Otherwise, ownership of cores in our ordered RA model will not be persistent.

Otherwise, whenever it cannot establish the Absorbing or Affine premise,
it will backtrack on the FromAssumption premise, causing a possible loop.
No idea why this happens, this may be a Coq bug...

Replace/remove some occurences of `persistently` into `persistent` where the property instead of the modality is used.

Rename `UCMRA` → `Ucmra`
Rename `CMRA` → `Cmra`
Rename `OFE` → `Ofe` (`Ofe` was already used partially, but many occurences were missing)
Rename `STS` → `Sts`
Rename `DRA` → `Dra`

I have reimplemented the tactic for introduction of ∀s/pures using type
classes, which directly made it much more modular.

The advantage is that we can directly use a Coq introduction pattern
`cpat` to perform actions to the pure assertion. Before, this had
to be done in several steps:

  iDestruct ... as "[Htmp ...]"; iDestruct "Htmp" as %cpat.

That is, one had to introduce a temporary name.

I expect this to be quite useful in various developments as many of
e.g. our invariants are written as:

  ∃ x1 .. x2, ⌜ pure stuff ⌝ ∗ spacial stuff.

This causes a bit of backwards incompatibility: it may now succeed with
later stripping below unlocked/TC transparent definitions. This problem
actually occured for `wsat`.

We used to normalize the goal, and then checked whether it was of
a certain shape. Since `uPred_valid P` normalized to `True ⊢ P`,
there was no way of making a distinction between the two, hence
`True ⊢ P` was treated as `uPred_valid P`.

In this commit, I use type classes to check whether the goal is of
a certain shape. Since we declared `uPred_valid` as `Typeclasses
Opaque`, we can now make a distinction between `True ⊢ P` and
`uPred_valid P`.

persistent context.

Given the source does not contain a box:

- Before: no-op if there is a Persistent instance.
- Now: no-op in all cases.

We now first iPoseProof the lemma and instantiate its premises before trying
to search for the sub-term where to apply. As a result, instantiation of the
premises of the applied lemmas happens only once, instead of it being done for
each sub-term as obtained by reshape_expr.

Fixes #96

This makes it easier to frame or introduce some modalities before introducing
universal quantifiers.

when using iCombine.