They are redundant because frac is discrete.

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).

In particular, remove ectx_language.atomic, because it seems unused
expect for a smart constructor for language.atomic.

* Values are considered as atomic expressions (this does not hurt, and
  this makes the proofs of atomicity simpler).

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

I know we don't use it.  Stating theorems also serves to document things, and IMHO this one is informative.  It also costs us nothing.

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

Fixes issue #20.

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

Based on an idea and WIP commits of J-H. Jourdan: the core of a CMRA
A is now a partial function A → option A.

TODO: define sum CMRA
TODO: remove one shot CMRA and define it in terms of sum

Rationale: to make the code closer to what is on paper, I want the notations
to look like quantifiers, i.e. have a binder built-in. I thus introduced the
following notations:

  [★ map] k ↦ x ∈ m, P
  [★ set] x ∈ X, P

The good thing - contrary to the notations that we had before that required an
explicit lambda - is that type annotations of k and x are now not printed
making goals much easier to read.

This gets rid of the (ambiguous) notation %l, because we can declare
LitLoc as a coercion. It also shortens the code.

It is not a library; it does not contain code, but instead is a core
part of heap_lang.

Also remove some superfluous map_ prefixes.