Change ToCss to take a CssWriter<W>

This more concrete wrapper type can write a prefix the very first time something
is written to it. This allows removing plenty of useless monomorphisations caused
by the former W/SequenceWriter<W> pair of types.

tests/unit/style/media_queries.rs

@@ -279,8 +279,7 @@ fn test_mq_expressions() {
 fn test_to_css() {
     test_media_rule("@media print and (width: 43px) { }", |list, _| {
         let q = &list.media_queries[0];
-        let mut dest = String::new();
-        assert_eq!(Ok(()), q.to_css(&mut dest));
+        let dest = q.to_css_string();
         assert_eq!(dest, "print and (width: 43px)");
     });
 }