Only use a local context when checking exit matchers.

This prevents context tree from deeper in the parse impacting exit checks for earlier in the parse. This is needed due to the trailing whitespace context element.

src/parser/list.rs

@@ -31,6 +31,12 @@ impl<T> List<T> {
         List { head: None }
     }
 
+    pub fn branch_from(trunk: &Rc<Node<T>>) -> Self {
+        List {
+            head: Some(trunk.clone()),
+        }
+    }
+
     pub fn push_front(&self, data: T) -> List<T> {
         List {
             head: Some(Rc::new(Node {