Make an Abstract Class for Ast Node to Support Generic Helper Function in Visitor

[aferr]

Make an Abstract Class for Ast Nodes which includes calls to Accept and make the elements of ast.h extend from this abstract class so that the abstract class can be referenced in a generic helper function in the implementation of the visitor. Context

Probably having an abstract class that defines the functions available to all AST nodes will help us implement other design patters beyond just this one.