Pass around event types as Atoms instead of Strings

`Event` internally stores the `type` as an `Atom`, and we're `String`s
everywhere, which can cause unnecessary allocations to occur since
they'll end up as `Atom`s anyways.

components/script/dom/htmltextareaelement.rs

@@ -238,7 +238,7 @@ impl HTMLTextAreaElement {
         let window = window_from_node(self);
         let window = window.r();
         let event = Event::new(GlobalRef::Window(window),
-                               DOMString::from("input"),
+                               atom!("input"),
                                EventBubbles::DoesNotBubble,
                                EventCancelable::NotCancelable);