Using this new definition we can express being contractive using a
Proper. This has the following advantages:

- It makes it easier to state that a function with multiple arguments
  is contractive (in all or some arguments).
- A solve_contractive tactic can be implemented by extending the
  solve_proper tactic.

    The idea on magic wand is to use it for curried lemmas and use ⊢ for uncurried lemmas.

Use COFEs only for the recursive domain equation solver

This reverts commit 2a7755fe because
it is no longer needed after Matthieu Sozeau reverted this change in
Coq 8.6. See also the discussion:

  [Coq-Club] Coq 8.6 typeclasses behavior change

at 11/16/2016 02:14 PM.

In Coq 8.6 type class search is not called recursively on premises
that are not type classes. To that end, we use a hint extern to invoke
an ordinary auto.

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.

This should make theme asier to parse, "{{{ v, v; l |-> v }}}" looks rather funny.

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

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.

This fact is deduced from reducibility. Unfortunately, this sometimes depends on the type of states being inhabited, so that this additional hypothesis sometimes appear.

rename program_logic.{ownership -> wsat}. It really is about world satisfaction and invariants more than about ownership.