refactoring of Heap. Use internal addPointsToEdge(). Field of a
FieldEdge can't be null, so removed check in removeNullObject()
remove not only nullObjectNodes and connected fieldEdges, but also
pointsToEdges to nullObjects
added test for access to return value of a called method, if the method
returns the globalObject
don't remove the GlobalObject from the Heap in
doFinalStuff()->collapseGlobalGraph()