And also rename the corresponding proof mode tactics.

Used in iRevert, iClear, iFrame, and for generalizing the IH in
iInduction and iLöb.

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

As proposed by JH Jourdan in issue 34.

Before, it failed when these tactics were invoked with persistent
hypotheses. The new behavior is more convenient when using these
tactics to build other tactics.

This comment mostly addresses issue #34.

There are still some issues:

- For iLöb we can write `iLöb (x1 .. xn) as "IH"` to revert x1 .. xn
  before performing Löb induction. An analogue notation for iInduction
  results in parsing conflicts.
- The names of the induction hypotheses in the Coq intro pattern are
  ignored. Instead, when using `iInduction x as pat "IH"` the induction
  hypotheses are given fresh names starting with "IH". The problem here
  is that the names in the introduction pattern are idents, whereas the
  induction hypotheses are inserted into the proof mode context, and thus
  need to have strings as names.

This closes issue 32.

This solves issue 33.

Before this commit, given "HP" : P and "H" : P -★ Q with Q persistent, one
could write:

  iSpecialize ("H" with "#HP")

to eliminate the wand in "H" while keeping the resource "HP". The lemma:

  own_valid : own γ x ⊢ ✓ x

was the prototypical example where this pattern (using the #) was used.

However, the pattern was too limited. For example, given "H" : P₁ -★ P₂ -★ Q",
one could not write iSpecialize ("H" with "#HP₁") because P₂ -★ Q is not
persistent, even when Q is.

So, instead, this commit introduces the following tactic:

  iSpecialize pm_trm as #

which allows one to eliminate implications and wands while being able to use
all hypotheses to prove the premises, as well as being able to use all
hypotheses to prove the resulting goal.

In the case of iDestruct, we now check whether all branches of the introduction
pattern start with an `#` (moving the hypothesis to the persistent context) or
`%` (moving the hypothesis to the pure Coq context). If this is the case, we
allow one to use all hypotheses for proving the premises, as well as for proving
the resulting goal.

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.

In most cases it is a temporary whose name is useless and just
clutters the error message.