Stop implementing Copy for JS<T>.

A copy of a JS<T> doesn't have the rooting properties of the original,
so it makes no sense for it to implement Copy.

components/script/dom/nodelist.rs

@@ -64,7 +64,7 @@ impl NodeListMethods for NodeList {
     fn Item(&self, index: u32) -> Option<Root<Node>> {
         match self.list_type {
             NodeListType::Simple(ref elems) => {
-                elems.get(index as usize).map(|node| Root::from_rooted(*node))
+                elems.get(index as usize).map(|node| node.root())
             },
             NodeListType::Children(ref list) => list.item(index),
         }