Introduction of polymorphic nodes in object trees.

* Some searches in object trees may return multiple nodes (e.g. "var.A.x" and "var.B.x" for "var.x")
* Only leaves in object trees can be the target of summary arcs.
* Polymorphic nodes aren't interconnected with data dependencies.
* ClassGraph's map now split into kinds of vertex.
* ClassGraph's vertex now generic.
* Polymorphic member nodes cannot be leaves in object trees.