Small tidy of pepa_ast.py

The asttype is passed in as an argument to the constructor for the AST classes. This means it is really just data, but then it is used more like something that is intended to be static and the same for all nodes of the same kind. So I've refactored this so that the asttype is stored within each class definition (such that it can be obtained via the class name) and used so that it is correctly set within the constructor for all AST instances.

allan