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.

We are now using the prefixes Into, From, and Is (the first two are
inspired by the names of some traits in the Rust stdlib), and hopefully
doing that consistenly.

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

This is more natural, match should be used in user code, not case.

Fixes issue #20.

This introduces n hypotheses and destructs the nth one.

We used => before, which is strange, because it has another meaning
in ssreflect.

be the same as .

This is a fairly intrusive change, but at least makes notations more
consistent, and often shorter because fewer parentheses are needed. Note
that viewshifts already had the same precedence as →.

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.

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.

And make constants P for which we do not want of_val P to reduce Opaque.

through definitions.