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/domtokenlist.rs

@@ -17,6 +17,7 @@ use servo_util::namespace::Null;
 use servo_util::str::{DOMString, HTML_SPACE_CHARACTERS};
 
 #[deriving(Encodable)]
+#[must_root]
 pub struct DOMTokenList {
     reflector_: Reflector,
     element: JS<Element>,