An attempt to make the search algorithm easier to understand.

- clear ownership: the SpecificBumpPtrAllocator owns all StateNodes
- this allows us to simplify the memoization data structure into a
  std::set (FIXME: figure out whether we want to use a hash based
  data structure).
- introduces StateNode as recursive data structure, instead of using
  Edge and the Seen-map combined to drill through the graph
- using a count to stabilize the penalty instead of relying on the
  container
- pulled out a method to forward-apply states in the end

This leads to a ~40% runtime decrease on Nico's benchmark.

Main FiXME is that the parameter lists of some function get too long.
I'd vote for either pulling the Queue etc into the Formatter proper,
or creating an inner class just for the search algorithm.