More AST refactoring -- each of the code wrapping classes just returns a resource from its evaluate() method, and all of the work is done in the evaluate_code method. This makes the code cleaner, because it means 1) evaluate() has the same prototype as all of the other AST classes, 2) evaluate() is no longer called indirectly through the Parser Resource class, and 3) the classes themselves are responsible for creating the resources, rather than it being done in the Compile class.