Separate style+layout and layout-specific wrapper functionality.

This patch does a number of things, unfortunately all at once:
* Hoists a large subset of the layout wrapper functionality into the style system.
* Merges TElementAttributes into the newly-created TElement.
* Reorganizes LayoutData by style vs layout, and removes LayoutDataShared.
* Simplifies the API for borrowing style/layout data.

There's still more to do to make the style system usable standalone, but
this is a good start.

components/style/data.rs

@@ -0,0 +1,48 @@
+/* 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/. */
+
+use properties::ComputedValues;
+use std::sync::Arc;
+use std::sync::atomic::AtomicIsize;
+
+pub struct PrivateStyleData {
+    /// The results of CSS styling for this node.
+    pub style: Option<Arc<ComputedValues>>,
+
+    /// The results of CSS styling for this node's `before` pseudo-element, if any.
+    pub before_style: Option<Arc<ComputedValues>>,
+
+    /// The results of CSS styling for this node's `after` pseudo-element, if any.
+    pub after_style: Option<Arc<ComputedValues>>,
+
+    /// Information needed during parallel traversals.
+    pub parallel: DomParallelInfo,
+}
+
+impl PrivateStyleData {
+    pub fn new() -> PrivateStyleData {
+        PrivateStyleData {
+            style: None,
+            before_style: None,
+            after_style: None,
+            parallel: DomParallelInfo::new(),
+        }
+    }
+}
+
+/// Information that we need stored in each DOM node.
+#[derive(HeapSizeOf)]
+pub struct DomParallelInfo {
+    /// The number of children that still need work done.
+    pub children_count: AtomicIsize,
+}
+
+impl DomParallelInfo {
+    pub fn new() -> DomParallelInfo {
+        DomParallelInfo {
+            children_count: AtomicIsize::new(0),
+        }
+    }
+}
+