Remove some unnecessary uses of as_slice

For the majority of these cases, `as_slice` can be removed due to
`Deref`. In particular, `Deref` for:

* `String` -> `str`
* `Atom` -> `str`

The latter of those two requires, a bump of the locked `string-cache`
library

components/script/dom/document.rs

@@ -323,7 +323,7 @@ impl<'a> DocumentHelpers<'a> for JSRef<'a, Document> {
             let node: JSRef<Node> = NodeCast::from_ref(element);
             node.is_in_doc()
         });
-        assert!(!id.as_slice().is_empty());
+        assert!(!id.is_empty());
 
         let mut idmap = self.idmap.borrow_mut();