Expand classes to avoid needing some rules

Some `RULES` pragmas are used to specialize functions to specific graph implementations. I believe most or all of these could be eliminated by adding the relevant functions to `Graph` or `DynGraph`. If you are concerned that some of them may not really belong in the API, and don't know what user-friendly methods to add to support them, you can hide those methods by defining the class in an "internal" module, and only export some of its methods. Using the class system instead of rules, when possible, guarantees that the preferred implementation will always be used.