Move AstNode into the types crate.

Now that it is used for more than just iteration, it makes sense to promote it to the types crate.

src/iter/all_ast_node_iter.rs

@@ -1,7 +1,7 @@
 use std::collections::VecDeque;
 
-use super::ast_node::AstNode;
 use super::ast_node_iter::AstNodeIter;
+use crate::types::AstNode;
 
 pub struct AllAstNodeIter<'r, 's> {
     root: Option<AstNode<'r, 's>>,