Fixed: nondeterminism resolution, star operator

* Golog2.resolve didn't propagate in Alt nodes.
* Kleene star operator is now a helper function which exploits
  infinite lists and lazy evaluation. The old construction using
  Prelude.iterate didn't work (well) anyway, because it created an
  Nondet [Nil, p, p;p, p;p;p, ...] node with an infinite list. This
  makes resolution of nondeterminism impossible at this node.
  Now we create a node Nondet [Nil, Nondet [p, p; Nondet [p, ...]]].
  With this tree structure we can evaluate the tree up to a certain
  depth and thus resolve nondeterminism.