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.

ports/geckolib/string_cache/Cargo.toml

@@ -14,5 +14,5 @@ cfg-if = "0.1.0"
 gecko_bindings = {version = "0.0.1", path = "../gecko_bindings"}
 heapsize = "0.3.5"
 libc = "0.2"
-selectors = "0.8"
+selectors = "0.9"
 serde = "0.8"