Added a walker class to replace ast.Visitor and ast.NodeTransformer
core/walker: added support for a dispatcher object as a callback
core/walker: checking for ignored node mutation during inspect(), walking the fields of a changed node
core/symbol_finder: using the new Walker
components/eliminate: using the new Walker
components/while_remover: using the new Walker
components/var_simplifier: using the new Walker (need to add the custom context support)