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And shorten the proof.

Notably, `big_andL_andL` and `big_andL_and` where a ⊣⊢ and ⊢ version
of the same lemma. I favored the `big_opL_op` naming scheme.

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.