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/counter_style_rule.rs

@@ -0,0 +1,12 @@
+/* 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/. */
+
+// TODO(emilio): unify this, components/style/counter_style.rs, and
+// components/style/gecko/rules.rs
+#![allow(missing_docs)]
+
+#[cfg(feature = "servo")]
+pub use counter_style::CounterStyleRuleData as CounterStyleRule;
+#[cfg(feature = "gecko")]
+pub use gecko::rules::CounterStyleRule;