(□ P) now means (bi_bare (bi_persistently P)).

This is motivated by the fact that these two modalities are rarely
used separately.

In the case of an affine BI, we keep the □ notation. This means that a
bi_bare is inserted each time we use □. Hence, a few adaptations need
to be done in the proof mode class instances.

Whenever we iSpecialize something whose conclusion is persistent, we now have
to prove all the premises under the sink modality. This is strictly more powerful,
as we now have to use just some of the hypotheses to prove the premises, instead of
all.

This also applies to the introduction pattern `!#`. Both will now
introduce as many ■ or □ as possible. This behavior is consistent
with the dual, `#`, which also gets rid of as many ■ and □ modalities
as possible.

(All the later lemmas are now prefixed by later_, and dito for
laterN, and except_0).

These unfolds kind of make sense, and I was quite surprised that
it used to work before. However, when changing to primitive
records, these unfolds are actually needed.

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...

As Aleš observed, in the ordered RA model it is not, unless the order on the
unit is timeless.

Thanks to discussions with Ales and Amin.