The `2: { ... }` syntax is not yet supported there.

This is in preparation for coq/coq#9274.

Fixes #156

Based on an earlier MR by @jung.

(□ 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.

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

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

This is to be used on top of stdpp's 4b5d254e.

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