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 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 is more consistent with the proofmode, where we also call it pure.

Fixes issue #20.

be the same as ↔.

This is a fairly intrusive change, but at least makes notations more
consistent, and often shorter because fewer parentheses are needed. Note
that viewshifts already had the same precedence as →.

It used to be: (P ={E}=> Q) := (True ⊢ (P → |={E}=> Q))
Now it is: (P ={E}=> Q) := (P ⊢ |={E}=> Q)

Also remove some superfluous map_ prefixes.

Thanks to Amin Timany for the suggestion.

Add both non-expansive and contractive functors, and bundle them for the general Iris instance as well as the global functor construction

This allows us to move the \later in the user-defined functor to any place we want.
In particular, we can now have "\later (iProp -> iProp)" in the ghost CMRA.

Since functor instances are just used as combinators, there is really
no need for functors that are not contractive.

This cleans up some ad-hoc stuff and prepares for a generalization
of saved propositions.

The performance gain seems neglectable, unfortunatelly...

And now the part that I forgot to commit.

Also, give all these global functors the suffix GF to avoid shadowing
such as we had with authF.

And add some type annotations for clarity.

I added a new typeclass "inGF" to witness that a particular *functor* is part of \Sigma. inG, in contrast, witnesses a particular *CMRA* to be in there, after applying the functor to "\later iProp".
inGF can be inferred if that functor is consed to the head of \Sigma, and it is preserved by consing a new functor to \Sigma. This is not the case for inG since the recursive occurence of \Sigma also changes.
For evry construction (auth, sts, saved_prop), there is an instance infering the respective authG, stsG, savedPropG from an inGF. There is also a global inG_inGF, but Coq is unable to use it.

I tried to instead have *only* inGF, since having both typeclasses seemed weird. However, then the actual type that e.g. "own" is about is the result of applying a functor, and Coq entirely fails to infer anything.

I had to add a few type annotations in heap.v, because Coq tried to use the "authG_inGF" instance before the A got fixed, and ended up looping and expanding endlessly on that proof of timelessness.
This does not seem entirely unreasonable, I was honestly surprised Coq was able to infer the types previously.

The singleton maps notation is now also more consistent with the
insert <[_ := _]> _ notation for maps.

* These type classes bundle an identifier into the global CMRA with a proof
  that the identifier points to the correct CMRA. Bundling allows us to get
  rid of many arguments everywhere.

* I have setup the type classes so that we no longer have to keep track of the
  global CMRA identifiers. These are implicit and resolved automatically.

* For heap I am also bundling the name of the heap RA instance. There always
  should be at most one heap instance so this does not introduce ambiguities.

* We now have a "maps to" notation!