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Note sure whether these premises are the weakest possible.

Also, persistent stuff goes before plain stuff.

We do not have a notation for `bi_affinely` either, so this is at
least consistent.

This gives a 25% speedup on some files (e.g. boxes).

This commit contains some hacks to work arround Coq issue #5699.

This commit requires Coq v8.7 together with

  https://github.com/coq/coq/pull/1006

(□ 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.