1. CLEANUP: Relocated edge/2, edge/3, edge_components/3.
2. ADDED: unsymmetric_edge/2
3. CLEANUP: Explicit export of library(ugraphs) predicates
            (modulo renamings).
4. ADDED: connected_component/2.
5. ADDED: graph type predicates (a lot!).
6. ADDED: in_degree/3, out_degree/3, neighbor/3.