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/canvasgradient.rs

@@ -52,7 +52,7 @@ impl<'a> CanvasGradientMethods for JSRef<'a, CanvasGradient> {
 
         self.stops.borrow_mut().push(CanvasGradientStop {
             offset: (*offset) as f64,
-            color: parse_color(color.as_slice()).unwrap_or(default_black),
+            color: parse_color(&color).unwrap_or(default_black),
         });
     }
 }