New benchmark, and a fix in `DefaultEvaluator`.
New benchmark, and a fix in `DefaultEvaluator`.

The new benchmark is a Heap, straight from Okasaki's PhD thesis.
Currently the specification is very weak ("implements a set").
The fix is the addition of a missing case (SubsetOf) in the evaluator.

epfl-lara/stainless,epfl-lara/stainless,epfl-lara/stainless,epfl-lara/stainless