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To do so, we have introduced the specialization patterns:

  =>[H1 .. Hn] and =>[-H1 .. Hn]

That generate a goal in which the view shift is preserved. These specialization
patterns can also be used for e.g. iApply.

Note that this machinery is not tied to primitive view shifts, and works for
various kinds of goal (as captured by the ToAssert type class, which describes
how to transform the asserted goal based on the main goal).

TODO: change the name of these specialization patterns to reflect this
generality.

Changes:
- We no longer have a different syntax for specializing a term H : P -★ Q whose
  range P or domain Q is persistent. There is just one syntax, and the system
  automatically determines whether either P or Q is persistent.
- While specializing a term, always modalities are automatically stripped. This
  gets rid of the specialization pattern !.
- Make the syntax of specialization patterns more consistent. The syntax for
  generating a goal is [goal_spec] where goal_spec is one of the following:

    H1 .. Hn : generate a goal using hypotheses H1 .. Hn
   -H1 .. Hn : generate a goal using all hypotheses but H1 .. Hn
           # : generate a goal for the premise in which all hypotheses can be
               used. This is only allowed when specializing H : P -★ Q where
               either P or Q is persistent.
           % : generate a goal for a pure premise.

We may want to do the same for iIntros ? and iDestruct "H" as {?} "H",
but that requires more work. However, I do not think I want to rely on
names chosen this way.

iSpecialize and iDestruct.

These tactics now all take an iTrm, which is a tuple consisting of
a.) a lemma or name of a hypotheses b.) arguments to instantiate
c.) a specialization pattern.

These tactics are superfluous:
- iPure H as pat => iDestruct H as pat
- iPersistent H => iSpecialize H "!"

It is no longer triggered when posing [P ⊢ Q] with [P] an evar. This, for
example, makes sure that iApply pvs_intro works, which failed before.

Now, it bases the type the quantifier ranges over on the goal, instead
of the witness. This works better when dealing with witnesses involving
type class constraints.

This reverts commit 3cc38ff6.

The reverted pure hypotheses and variables appear in the wrong order.