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]).

Fix typo in is_lock_proper for heap_lang.

See merge request !57

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.