Implement iterator for context.

src/context/list.rs

@@ -27,6 +27,10 @@ impl<'parent, T> List<'parent, T> {
     pub fn iter(&self) -> Iter<'_, T> {
         Iter { next: Some(self) }
     }
+
+    pub fn iter_list(&self) -> IterList<'_, T> {
+        Iter { next: Some(self) }
+    }
 }
 
 pub trait ListType<'parent, T> {
@@ -64,3 +68,17 @@ impl<'a, T> Iterator for Iter<'a, T> {
         ret
     }
 }
+
+pub struct IterList<'a, T> {
+    next: Link<'a, T>,
+}
+
+impl<'a, T> Iterator for IterList<'a, T> {
+    type Item = &'a List<'a, T>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let ret = self.next;
+        self.next = self.next.map(|this| this.get_parent()).flatten();
+        ret
+    }
+}