Use COFEs only for the recursive domain equation solver

The old name didn't make much sense. Also now we can have
pure_False too.

That range includes tabs and new lines.

Thanks Morten for spotting this problem.

This way we avoid the env_cbv tactic unfolding string related stuff
that appears in the goal and hypotheses of the proof mode.

The old choice for ★ was a arbitrary: the precedence of the ASCII asterisk *
was fixed at a wrong level in Coq, so we had to pick another symbol. The ★ was
a random choice from a unicode chart.

The new symbol ∗ (as proposed by David Swasey) corresponds better to
conventional practise and matches the symbol we use on paper.

That's like what we are doing when instantiating an arrow or wand.

This is more consistent with our current consensus of not using
implication. Also, it allows one to reintroduce the persistent
hypothesis into the spatial context.

Since env_cbv does not unfold these apps, we should do it ourselves,
to avoid ending up with partially evaluated terms.

This fixes issue 39.

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.

And also rename the corresponding proof mode tactics.

Used in iRevert, iClear, iFrame, and for generalizing the IH in
iInduction and iLöb.

This way we can use uPred_valid for validity of uPreds, which
more sense.

As proposed by JH Jourdan in issue 34.