Rewrite the style sharing candidate cache.

The style candidate cache had regressed a few times (see #12534), and my
intuition is that being able to disable all style sharing with a single rule in
the page is really unfortunate.

This commit redesigns the style sharing cache in order to be a optimistic cache,
but then reject candidates if they match different sibling-affecting selectors
in the page, for example.

So far the numbers have improved, but not so much as I'd wanted (~10%/20% of
non-incremental restyling time in general). The current implementation is really
dumb though (we recompute and re-match a lot of stuff), so we should be able to
optimise it quite a bit.

I have different ideas for improving it (that may or may not work), apart of the
low-hanging fruit like don't re-matching candidates all the time but I have to
measure the real impact.

Also, I need to verify it against try.

components/script/layout_wrapper.rs

@@ -444,6 +444,11 @@ impl<'le> TElement for ServoLayoutElement<'le> {
     }
 }
 
+impl<'le> PartialEq for ServoLayoutElement<'le> {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_node() == other.as_node()
+    }
+}
 
 impl<'le> ServoLayoutElement<'le> {
     fn from_layout_js(el: LayoutJS<Element>) -> ServoLayoutElement<'le> {