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/style/animation.rs

@@ -502,6 +502,7 @@ fn compute_style_for_animation_step(context: &SharedStyleContext,
             // as existing browsers don't appear to animate visited styles.
             let computed =
                 properties::apply_declarations(context.stylist.device(),
+                                               previous_style.rules(),
                                                iter,
                                                previous_style,
                                                previous_style,