First steps of &JSRef -> JSRef conversion

Replace &JSRef with JSRef in the bulk of the generated code. This will
remove a level of indirection throughout all DOM code.

This patch doesn't change methods implemented on JSRef<T> to take `self`
rather than `&self`, and it leaves a few other uses of &JSRef, but those
changes can be made incrementally.

components/script/dom/urlsearchparams.rs

@@ -56,7 +56,7 @@ impl URLSearchParams {
             },
             None => {}
         }
-        Ok(Temporary::from_rooted(&*usp))
+        Ok(Temporary::from_rooted(*usp))
     }
 }