auto merge of #4173 : Manishearth/servo/a-more-dom-struct, r=kmcallister

Now `#[dom_struct]` also generates Reflectable impls, and there's another lint to ensure that a DOM struct only contains one bare DOM field (as the first field) or a Reflector.

A lot of this was generated by sed -- each autogenerated change has its own commit for easy review; these will be squashed later.

components/script/dom/testbinding.rs

@@ -307,8 +307,3 @@ impl TestBinding {
     pub fn ReceiveVoidStatic() {}
 }
 
-impl Reflectable for TestBinding {
-    fn reflector<'a>(&'a self) -> &'a Reflector {
-        &self.reflector
-    }
-}