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/domimplementation.rs

@@ -53,7 +53,7 @@ impl DOMImplementation {
 impl<'a> DOMImplementationMethods for JSRef<'a, DOMImplementation> {
     // http://dom.spec.whatwg.org/#dom-domimplementation-createdocumenttype
     fn CreateDocumentType(self, qname: DOMString, pubid: DOMString, sysid: DOMString) -> Fallible<Temporary<DocumentType>> {
-        match xml_name_type(qname.as_slice()) {
+        match xml_name_type(&qname) {
             // Step 1.
             InvalidXMLName => Err(InvalidCharacter),
             // Step 2.