style: Split stylesheets.rs

This file has become quite bloated lately. This commit deletes that file in
favor of a set of submodules.

The only noticeable change apart from code move, is converting deep_clone_foo
methods into a trait.

It also unifies logic related to different style rules in the same place.

There's some missing work, specially related to font-face and counter-style, but
I think this is worth landing in the meantime.

components/style/stylesheets/namespace_rule.rs

@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/. */
+
+//! The `@namespace` at-rule.
+
+use {Namespace, Prefix};
+use cssparser::SourceLocation;
+use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
+use std::fmt;
+
+/// A `@namespace` rule.
+#[derive(Clone, Debug, PartialEq)]
+#[allow(missing_docs)]
+pub struct NamespaceRule {
+    /// The namespace prefix, and `None` if it's the default Namespace
+    pub prefix: Option<Prefix>,
+    /// The actual namespace url.
+    pub url: Namespace,
+    /// The source location this rule was found at.
+    pub source_location: SourceLocation,
+}
+
+impl ToCssWithGuard for NamespaceRule {
+    // https://drafts.csswg.org/cssom/#serialize-a-css-rule CSSNamespaceRule
+    fn to_css<W>(&self, _guard: &SharedRwLockReadGuard, dest: &mut W) -> fmt::Result
+    where W: fmt::Write {
+        dest.write_str("@namespace ")?;
+        if let Some(ref prefix) = self.prefix {
+            dest.write_str(&*prefix.to_string())?;
+            dest.write_str(" ")?;
+        }
+
+        // FIXME(emilio): Pretty sure this needs some escaping, or something?
+        dest.write_str("url(\"")?;
+        dest.write_str(&*self.url.to_string())?;
+        dest.write_str("\");")
+    }
+}