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_interface/lib.rs

@@ -65,7 +65,7 @@ pub struct StyleData {
 impl StyleData {
     pub fn new() -> Self {
         Self {
-            element_data: AtomicRefCell::new(ElementData::new(None)),
+            element_data: AtomicRefCell::new(ElementData::default()),
             parallel: DomParallelInfo::new(),
         }
     }