This regression was caused by a bug in handling spec patterns.

Fixed by stdpp 93b4ec70e13a573a9055a5bf1269f5885e18e843.

Assertions should have type sbi_car PROP and not bi_car (sbi_bi PROP).

This reverts commit 78ba9509.

This is to make sure that e.g. `//` in `iEval (rewrite .. // ..)` does
not immediately close the goal by reflexivity.

The types of propositions for monPred lemma need to be [monPred I PROP] and not [bi_car (monPredI I PROP)], otherwise iIntoValid fails in a very weird way. Seems to be related to a Coq bug.

This reverts commit 913059d2.

This reverts commit 849abb8d, reversing changes
made to 8de0b894.  The merge was done
accidentally, before review happened.

I saw no need for `stuckness_flip`: strong atomicity always works,
while weak atomicity works only for expressions that are not stuck.
Since this seemed unclear, I split lemma `wp_atomic'` up into
`wp_strong_atomic` (parametric in the WP's `s`) and `wp_weak_atomic`
(not). The proof mode instance is stated in terms of the derived rule
`wp_atomic` (parametric in `s`).