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/values/mod.rs

@@ -13,9 +13,9 @@ pub use cssparser::{RGBA, Token, Parser, serialize_identifier, CowRcStr, SourceL
 use parser::{Parse, ParserContext};
 use selectors::parser::SelectorParseErrorKind;
 #[allow(unused_imports)] use std::ascii::AsciiExt;
-use std::fmt::{self, Debug};
+use std::fmt::{self, Debug, Write};
 use std::hash;
-use style_traits::{ToCss, ParseError, StyleParseErrorKind};
+use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss};
 
 pub mod animated;
 pub mod computed;
@@ -34,8 +34,9 @@ define_keyword_type!(Auto, "auto");
 define_keyword_type!(Normal, "normal");
 
 /// Serialize a normalized value into percentage.
-pub fn serialize_percentage<W>(value: CSSFloat, dest: &mut W)
-    -> fmt::Result where W: fmt::Write
+pub fn serialize_percentage<W>(value: CSSFloat, dest: &mut CssWriter<W>) -> fmt::Result
+where
+    W: Write,
 {
     (value * 100.).to_css(dest)?;
     dest.write_str("%")
@@ -109,7 +110,10 @@ impl CustomIdent {
 }
 
 impl ToCss for CustomIdent {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
         serialize_identifier(&self.0.to_string(), dest)
     }
 }
@@ -180,7 +184,10 @@ impl Parse for KeyframesName {
 }
 
 impl ToCss for KeyframesName {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
         match *self {
             KeyframesName::Ident(ref ident) => ident.to_css(dest),
             KeyframesName::QuotedString(ref atom) => atom.to_string().to_css(dest),