Add unrooted_must_root lint for enums and structs containing JS<T>, as well as functions with JS<T> in their parameter list

For safe wrappers over JS<T> (eg Temporary<T>) use #[allow(unrooted_must_root)].
For all other types containing a #[must_root] value, annotate the type with #[must_root] to ensure that it is never used unrooted

components/script/dom/bindings/global.rs

@@ -34,6 +34,7 @@ pub enum GlobalRoot<'a, 'b> {
 /// A traced reference to a global object, for use in fields of traced Rust
 /// structures.
 #[deriving(Encodable)]
+#[must_root]
 pub enum GlobalField {
     WindowField(JS<Window>),
     WorkerField(JS<WorkerGlobalScope>),