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/layout/incremental.rs

@@ -47,6 +47,7 @@ bitflags! {
     }
 }
 
+
 impl RestyleDamage {
     /// Supposing a flow has the given `position` property and this damage, returns the damage that
     /// we should add to the *parent* of this flow.