This makes type checking more directed, and somewhat more predictable.

On the downside, it makes it impossible to declare the singleton on
lists as an instance of SingletonM and the insert and alter operations
on functions as instances of Alter and Insert. However, these were not
used often anyway.

This way, it won't pick arbitrary (and possibly wrong!) inG instances
when multiple ones are available. We achieve this by declaring:

  Hint Mode inG - - +

So that type class inference only succeeds when the type of the ghost
variable does not include any evars.

This required me to make some minor changes throughout the whole
development making some types explicit.

In order to improve flexibility, I have reverted the changes in commit
5cabd278 to counter, lock and spawn because an explicit namespace may be in
the way of modularity, for example, if the interfaces of these libraries will
expose namespaces through view shifts in the future.

Exposure of namespaces in the case of par is very unlikely, so to that end,
par remains to use an explicit namespace.

I also reverted 7952bca4 since there is no need for atomic to be a
boolean predicate anymore. Moreover, I introduced a hint database
fsaV for solving side-conditions related to FSAs, in particular,
side-conditions related to expressions being atomic.

We reify to a representation of expressions that includes an explicit
constructor for closed terms. Substitution can then be implemented as
the identify, which enables us to perform it using computation.

The intropattern {H} also meant clear (both in ssreflect, and the logic
part of the introduction pattern).

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.

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.

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)

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.

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.