Freshness manager optimization

[berpeti]

To solve freshness constraints (i.e., x \notin free_evars \phi), the freshness manager should not expand all conditions. One option for the optimization could be the following:

1. Deconstruct the goal into a list of subgoals for the metavariables. For example, x \notin \phi $ \psi could be deconstructed to x \notin \phi and x \notin \psi.
2. Solve the subgoals by using the fact that the metavariables are included in the freshness manager (by using the index_of tactic).