Use `ε` for CMRA unit

See merge request !62

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]).

For obsolete reasons, that no longer seem to apply, we used ∅ as the
unit.

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.