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.

components/style/properties/shorthand/serialize.mako.rs

@@ -2,15 +2,20 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use style_traits::ToCss;
+use style_traits::{CssWriter, ToCss};
 use values::specified::{BorderStyle, Color};
-use std::fmt;
+use std::fmt::{self, Write};
 
-fn serialize_directional_border<W, I,>(dest: &mut W,
-                                       width: &I,
-                                       style: &BorderStyle,
-                                       color: &Color)
-    -> fmt::Result where W: fmt::Write, I: ToCss {
+fn serialize_directional_border<W, I,>(
+    dest: &mut CssWriter<W>,
+    width: &I,
+    style: &BorderStyle,
+    color: &Color,
+) -> fmt::Result
+where
+    W: Write,
+    I: ToCss,
+{
     width.to_css(dest)?;
     dest.write_str(" ")?;
     style.to_css(dest)?;