Add triple notation for generalized post-condition

This changeset defines notation for the Iris style of writing Hoare triples:

`{{{ P }}} e {{{ v1 .. vn; T, Q }}} := P ★ (∀ v1 .. vn, Q → Φ T) ⊢ WP e {{ Φ }}`

For no good reason the notation is parsing only, although I do not declare it as such. We might want to do that though, since it might be too hard to understand a Hoare triple goal without unfolding it.

I have changed the barrier specifications to use the new notation in an attempt to demonstrate their usefulness (or, at a minimum, their applicability). The changes are rather minimal, as you can see.

## Changes

First and foremost, the specifications change. (Duh!)
Then, there are three kinds of changes to the proofs:
1. The first `iIntros` needs to take care of introducing `Φ`. No big deal, in my opinion.
2. Introducing the spatial assumptions needs one additional level of structure since we go from

    ```P1 ★ P2 ★ (∀ v, Q v -★ Φ v)```
to
   ```(P1 ★ P2) ★ (∀ v, Q v -★ Φ v)```

3. A post-condition of `True` leads to the rather annoying hypothesis `True -★ Φ v`, which (as far as I can tell) cannot be made to behave the same as just (Φ v) in the context of `iFrame`.

## Applicability

I have also looked at most other examples of specifications in heap_lang/lib.  The notation seems to be applicable to almost all of them. The only place where I spotted an obvious mismatch is par.v, where the current lemmas have a later before the generalized post-condition, as in `... ★ (∀ .., ... -★ ▷ Φ ..) ⊢ WP ..`. We could always add another pair of notations for this special case, I suppose.

## Nomenclature
I think "Texan triple" would be a good name, seeing how everything is bigger in Texas, including the number of curly braces.

See merge request !9