Move match and cascade temporaries to CurrentElementInfo

Before this change, the `ComputedStyle` struct that is part of permanent style
data per element holds 2 `StrongRuleNode`s (unvisited and visited) and 2
`Arc<ComputedValues>` (unvisited and visited).

Both rule nodes and the visited values don't actually need to be here.  This
patch moves these 3 to new temporary storage in `CascadeInputs` on
`CurrentElementInfo` during the match and cascade process.  Rule nodes are
pushed down inside the `ComputedValues` for later access after the cascade.
(Visited values were already available there.)

The permanent style data per element now has just the `Arc<ComputedValues>` for
itself and eager pseudo-elements (plus the `RestyleHint`).

MozReview-Commit-ID: 3wq52ERMpdi

components/script/layout_wrapper.rs

@@ -902,7 +902,7 @@ impl<'ln> ThreadSafeLayoutNode for ServoThreadSafeLayoutNode<'ln> {
     fn parent_style(&self) -> Arc<ComputedValues> {
         let parent = self.node.parent_node().unwrap().as_element().unwrap();
         let parent_data = parent.get_data().unwrap().borrow();
-        parent_data.styles().primary.values().clone()
+        parent_data.styles.primary().clone()
     }
 
     fn debug_id(self) -> usize {