Generalize update tactics into iMod and iModIntro for modalities.
There are now two proof mode tactics for dealing with modalities: - `iModIntro` : introduction of a modality - `iMod pm_trm as (x1 ... xn) "ipat"` : eliminate a modality The behavior of these tactics can be controlled by instances of the `IntroModal` and `ElimModal` type class. We have declared instances for later, except 0, basic updates and fancy updates. The tactic `iMod` is flexible enough that it can also eliminate an updates around a weakest pre, and so forth. The corresponding introduction patterns of these tactics are `!>` and `>`. These tactics replace the tactics `iUpdIntro`, `iUpd` and `iTimeless`. Source of backwards incompatability: the introduction pattern `!>` is used for introduction of arbitrary modalities. It used to introduce laters by stripping of a later of each hypotheses.