Added DataStructure.hs

DataStructure contains the DataStructure and Retrievable type classes,
as well as efficient Stack and Queue datatypes.
Added Search.hs for generic tree search.
Added ConstraintGraph.hs.

ConstraintGraph models models variable domains and directed, binary
constraints on variables. It is not an actual graph, but a
variable-indexed map of constraints.
Removed SimpleCSP.hs, added CSP.hs

CSP .hs can solve CSP problems with binary constraints and static
variable ordering.

SimpleCSP contained no functionality and was thus removed.