extensibility of eval

With the way eval is currently structured, it is not extensible. This can be fixed by adding an eval method to App and its subclasses and having the App classes handle opcodes specific to them. A fringe benefit here is that we get to break up eval into multiple large methods rather than having one huge method on PNode and another on CNode.

And that extensibility is key to keeping COODBMS useful for more than just the AgileWiki project.