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/attr.rs

@@ -78,7 +78,7 @@ impl Str for AttrValue {
             AttrValue::String(ref value) |
                 AttrValue::TokenList(ref value, _) |
                 AttrValue::UInt(ref value, _) => &value,
-            AttrValue::Atom(ref value) => value.as_slice(),
+            AttrValue::Atom(ref value) => &value,
         }
     }
 }