Change the way we handle view shifts in post-conditions

Merged Ralf Jung requested to merge ralf/texan into master

Now we try to avoid adding them unnecessarily, so we don't have to remove them automatically any more.

The overall tally in the proofs (i.e., excluding changes in proof mode and lifting lemmas) is: 14 removed iModIntro (and equivalent tactics), 7 insertions of wp_fupd. So it seems we actually more often do not need that final update than we do need it. Not to mention this also simplifies the lifting lemmas and the proof mode, doing less unnecessary work (adding updates and then removing them again).

On the minus side, if the update is missing, unexperienced users will have a hard time figuring out what to do. The change typically needs to be made at the beginning of the proof, the problem only surfaces at the end. This could be mitigated by providing a tactic for proving texan triples that does the wp_fupd (and the introducing the \Phi). While this would re-add most of the 14 removed iModIntro, we could still keep the simplified lifting lemmas and proof mode.

Cc @robbertkrebbers @jjourdan what do you think?

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