And also rename the corresponding proof mode tactics.

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

This closes issue 32.

Following the time anology of later, the step-index 0 corresponds does
not correspond to 'now', but rather to the end of time (i.e. 'last').

This is allowed as long as one of the conjuncts is thrown away (i.e. is a
wildcard _ in the introduction pattern). It corresponds to the principle of
"external choice" in linear logic.

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).

Prove some properties about it, and define timeless in terms of it,
and factor this notion out of raw view shifts.

It not behaves more consistently with iExact and thus also works in the
case H : P -★ □^n Q |- Q.

This avoids recompilation of coq_tactics each time an instance is added.

That way type class search becomes more predictable.

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.