Generic Problem Instances

[dharabor]

Currently we typedef problem_instance as warthog::problem_instance<pack_id>. This means all problem instances for any domain must conform to this specification, even if their internal state descriptions are different. The problematic typedef appears as part of the API of unidirectional_search and perhaps other places. We should change it to allow different types of domain-specific problem_instances to be created and used by the search algorithm.

Open question: how can we achieve this goal without complicating the API of e.g., unidirectional_search? Inheritance from a base problem_instance seems an obvious approach. Can we do it another way?