unrooted_must_root lint misses Option<JS<T>> return values

[jdm]

This is unfortunate.

cc @Manishearth

[Manishearth]

Ah, this is because we don't check return values at all.

[Manishearth]

...turns out the entire function checking lint was entirely broken when I updated things to use ty::Ty instead of ast::Ty.

[Manishearth]

/home/manishearth/Mozilla/voser/components/script/dom/htmlcanvaselement.rs:38:23: 38:28 note: in this expansion of #[derive_Clone] (defined in /home/manishearth/Mozilla/voser/components/script/dom/htmlcanvaselement.rs)
/home/manishearth/Mozilla/voser/components/script/dom/node.rs:1374:52: 1374:56 error: Type must be rooted, #[deny(unrooted_must_root)] on by default
/home/manishearth/Mozilla/voser/components/script/dom/node.rs:1374     pub fn new_without_doc(type_id: NodeTypeId) -> Node {
                                                                                                                      ^~~~
/home/manishearth/Mozilla/voser/components/script/dom/node.rs:1378:61: 1378:65 error: Type must be rooted, #[deny(unrooted_must_root)] on by default
/home/manishearth/Mozilla/voser/components/script/dom/node.rs:1378     fn new_(type_id: NodeTypeId, doc: Option<&Document>) -> Node {
                                                                                                                               ^~~~
/home/manishearth/Mozilla/voser/components/script/dom/servohtmlparser.rs:46:40: 46:60 error: Type must be rooted, #[deny(unrooted_must_root)] on by default
/home/manishearth/Mozilla/voser/components/script/dom/servohtmlparser.rs:46     pub fn get_or_create(&self, child: NodeOrText<JS<Node>>) -> Root<Node> {
                                                                                                                   ^~~~~~~~~~~~~~~~~~~~
/home/manishearth/Mozilla/voser/components/script/dom/webglrenderingcontext.rs:150:53: 150:77 error: Type must be rooted, #[deny(unrooted_must_root)] on by default
/home/manishearth/Mozilla/voser/components/script/dom/webglrenderingcontext.rs:150     pub fn bound_texture_for(&self, target: u32) -> Option<JS<WebGLTexture>> {
                                                                                                                                       ^~~~~~~~~~~~~~~~~~~~~~~~
/home/manishearth/Mozilla/voser/components/script/page.rs:122:36: 122:49 error: Type must be rooted, #[deny(unrooted_must_root)] on by default
/home/manishearth/Mozilla/voser/components/script/page.rs:122     pub fn set_frame(&self, frame: Option<Frame>) {
                                                                                                 ^~~~~~~~~~~~~
time: 3.006; rss: 758MB lint checking

Will finish fixing these later today

[jdm]

Perhaps I'll try to interest someone in #5646 to avoid this problem occurring again.