Removed ability to customize starting node and edge sequences.
Nodes/edges are forward-declared, so don't need to use asterisk methods.
GraphEdge objects now support the read-only subset of Relational, which will allow for cool polymorphism (w/r/t relation naming) in designing procedures to traverse graphs.
Refactored GraphNode code to split out the creation of a hash-map into an external procedure, just as GraphEdges already did.
Edge relations are stored in edges-relations as sets, not bijections.