ast: Add Visitor interface

Questions: do we need this?

src/ast.rs

@@ -2,6 +2,8 @@
 
 pub mod macro_matcher;
 
+pub mod visit;
+
 /// A value with an annotation.
 #[derive(Clone, PartialEq, Eq)]
 pub struct Anno<T: Annotation, A: Annotation = Span>(pub T, pub A);