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/style/dom.rs

@@ -201,7 +201,7 @@ pub trait PresentationalHintsSynthetizer {
         where V: Push<DeclarationBlock<Vec<PropertyDeclaration>>>;
 }
 
-pub trait TElement : Sized + Copy + Clone + ElementExt + PresentationalHintsSynthetizer {
+pub trait TElement : PartialEq + Sized + Copy + Clone + ElementExt + PresentationalHintsSynthetizer {
     type ConcreteNode: TNode<ConcreteElement = Self, ConcreteDocument = Self::ConcreteDocument>;
     type ConcreteDocument: TDocument<ConcreteNode = Self::ConcreteNode, ConcreteElement = Self>;