This generalization is surprisingly easy in Iris 3.0, so I could not
resist not doing it :).

This makes stuff more uniform and also removes the need for the [inGFs]
type class. Instead, there is now a type class [subG Σ1 Σ2] which expresses
that a list of functors [Σ1] is contained in [Σ2].

This better reflects the name of the bind rule.

I renamed an internal tactic that was previously called wp_bind into
wp_bind_core.

Use it to prove that tests/barrier_client and tests/heap_lang are adequate.

Also make those for introduction and elimination more symmetric:

  !%   pure introduction         %        pure elimination
  !#   always introduction       #        always elimination
  !>   later introduction        > pat    timeless later elimination
  !==> view shift introduction   ==> pat  view shift elimination

This commit features:

- A simpler model. The recursive domain equation no longer involves a triple
  containing invariants, physical state and ghost state, but just ghost state.
  Invariants and physical state are encoded using (higher-order) ghost state.

- (Primitive) view shifts are formalized in the logic and all properties about
  it are proven in the logic instead of the model. Instead, the core logic
  features only a notion of raw view shifts which internalizing performing frame
  preserving updates.

- A better behaved notion of mask changing view shifts. In particular, we no
  longer have side-conditions on transitivity of view shifts, and we have a
  rule for introduction of mask changing view shifts |={E1,E2}=> P with
  E2 ⊆ E1 which allows to postpone performing a view shift.

- The weakest precondition connective is formalized in the logic using Banach's
  fixpoint. All properties about the connective are proven in the logic instead
  of directly in the model.

- Adequacy is proven in the logic and uses a primitive form of adequacy for
  uPred that only involves raw views shifts and laters.

Some remarks:

- I have removed binary view shifts. I did not see a way to describe all rules
  of the new mask changing view shifts using those.
- There is no longer the need for the notion of "frame shifting assertions" and
  these are thus removed. The rules for Hoare triples are thus also stated in
  terms of primitive view shifts.

TODO:

- Maybe rename primitive view shift into something more sensible
- Figure out a way to deal with closed proofs (see the commented out stuff in
  tests/heap_lang and tests/barrier_client).

This way type class inference is not invokved when used in tactics
like iPvs while not having to write an @.

(Idea suggested by Ralf.)

This way, it won't pick arbitrary (and possibly wrong!) inG instances
when multiple ones are available. We achieve this by declaring:

  Hint Mode inG - - +

So that type class inference only succeeds when the type of the ghost
variable does not include any evars.

This required me to make some minor changes throughout the whole
development making some types explicit.

This reverts commit c43eb936.

The hack using class_apply has some strange behaviors, see:

  https://sympa.inria.fr/sympa/arc/coq-club/2016-07/msg00094.html

We reify to a representation of expressions that includes an explicit
constructor for closed terms. Substitution can then be implemented as
the identify, which enables us to perform it using computation.

The intropattern {H} also meant clear (both in ssreflect, and the logic
part of the introduction pattern).

For example iIntros "{$H1 H2} H1" frames H1, clears H2, and introduces H1.

This fixes a bug in 916ff44a causing proof mode notations not being
pretty printed.

Concretely, when execution of any of the wp_ tactics does not yield
another wp, it will make sure that a view shift is kept. This
behavior was already partially there, but now it is hopefully more
consistent.

This tweak allows us to declare pvs as an instance of FromPure (it is not
an instance of IntoPure), making some tactics (like iPureIntro and done)
work with pvs too.

This is more natural, match should be used in user code, not case.

Fun test case to play arround with abstract predicates.

Fixes issue #20.

This introduces n hypotheses and destructs the nth one.