Make DOMString a newtype around String, rather than a typedef.

This should make it somewhat easier to experiment with alternative
representations in the future. To reduce churn, this commit leaves the String
field public, though.

Also, this will allow us to use the default String type to represent the IDL
USVString type, which explicitly forbids unpaired surrogates, ans as such is
a better match to the Rust String type.

components/script/dom/css.rs

@@ -20,7 +20,7 @@ impl CSS {
             return Err(Error::InvalidCharacter);
         }
         let mut escaped = DOMString::new();
-        serialize_identifier(&ident, &mut escaped).unwrap();
+        serialize_identifier(&ident, &mut escaped.0).unwrap();
         Ok(escaped)
     }
 }