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/import_rule.rs

@@ -0,0 +1,59 @@
+/* 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 [`@import`][import] at-rule.
+//!
+//! [import]: https://drafts.csswg.org/css-cascade-3/#at-import
+
+use cssparser::SourceLocation;
+use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
+use std::fmt;
+use style_traits::ToCss;
+use stylearc::Arc;
+use stylesheets::stylesheet::Stylesheet;
+use values::specified::url::SpecifiedUrl;
+
+/// The [`@import`][import] at-rule.
+///
+/// [import]: https://drafts.csswg.org/css-cascade-3/#at-import
+#[derive(Debug)]
+pub struct ImportRule {
+    /// The `<url>` this `@import` rule is loading.
+    pub url: SpecifiedUrl,
+
+    /// The stylesheet is always present.
+    ///
+    /// It contains an empty list of rules and namespace set that is updated
+    /// when it loads.
+    pub stylesheet: Arc<Stylesheet>,
+
+    /// The line and column of the rule's source code.
+    pub source_location: SourceLocation,
+}
+
+impl Clone for ImportRule {
+    fn clone(&self) -> ImportRule {
+        let stylesheet: &Stylesheet = &*self.stylesheet;
+        ImportRule {
+            url: self.url.clone(),
+            stylesheet: Arc::new(stylesheet.clone()),
+            source_location: self.source_location.clone(),
+        }
+    }
+}
+
+impl ToCssWithGuard for ImportRule {
+    fn to_css<W>(&self, guard: &SharedRwLockReadGuard, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        dest.write_str("@import ")?;
+        self.url.to_css(dest)?;
+        let media = self.stylesheet.media.read_with(guard);
+        if !media.is_empty() {
+            dest.write_str(" ")?;
+            media.to_css(dest)?;
+        }
+        dest.write_str(";")
+    }
+}