This problem has been reported by Léon Gondelman.

Before, when using, for example wp_alloc, in an expression like:

  ref (ref v)

It would apply `tac_wp_alloc` to the outermost ref, after which it
fails to establish that the argument `ref v` is a value. In this
commit, other evaluation positions will be tried whenever it turn
out that the argument of the construct is not a value. The same
applies to store/cas/...

I have implemented this by making use of the new `IntoVal` class.

Expression `e` such that `to_val e = Some v` is in the context gets
reflected into value `v` together with the proof that `to_val e = Some
v`.

This is helpful for substitution and for `solve_to_val` operating on
the reflected syntax.

This way `IntoLaterNEnvs` is ought to be computed less frequently

The tactic was doing something weird and only once used.

- Get rid of wp_finish, which was a hack.
- Write the wp_ tactics for stateful steps in the same style as
  wp_pure, i.e. by taking the context into account.
- Make use of the context K in wp_pure.

Instead of writing a separate tactic lemma for each pure reduction,
there is a single tactic lemma for performing all of them.

The instances of PureExec can be shared between WP tactics and, e.g.
symbolic execution in the ghost  threadpool

Typeclass search gets less confused when this version is used, also,
we had the same for `wp_bind` already.

In order to do that, we need to quantify over non-expansive
predicates instead of arbitrary predicates.

That caused some problems, e.g.:

  From iris.base_logic Require Export fix.

Gave:

  Syntax error: [constr:global] expected after
  [export_token] (in [vernac:gallina_ext]).

persistent context.

Given the source does not contain a box:

- Before: no-op if there is a Persistent instance.
- Now: no-op in all cases.

We now first iPoseProof the lemma and instantiate its premises before trying
to search for the sub-term where to apply. As a result, instantiation of the
premises of the applied lemmas happens only once, instead of it being done for
each sub-term as obtained by reshape_expr.

- Use Φ and Ψ for predicates.
- Use _1 and _2 suffixes for the different directions of a lemma.
- Not all lemmas started with _uPred; we do not let the bigop lemmas (for instance)
  start with uPred_ either, so I just got rid of the prefix.

Fixes #96