selectors: Add parsing support for ::slotted().

Without turning it on yet, of course.

The reason why I didn't use the general PseudoElement mechanism is because this
pseudo is a bit of its own thing, and I found easier to make ::selectors know
about it (because you need to jump to the assigned slot) than the other way
around.

Also, we need to support ::slotted(..)::before and such, and supporting multiple
pseudo-elements like that breaks some other invariants around the SelectorMap,
and fixing those would require special-casing slotted a lot more in other parts
of the code.

Let me know if you think otherwise.

I also don't like much the boolean tuple return value, but I plan to do some
cleanup in the area in a bit, so it should go away soon, I'd hope.

components/selectors/tree.rs

@@ -84,6 +84,13 @@ pub trait Element: Sized + Clone + Debug {
     /// Whether this element is a `link`.
     fn is_link(&self) -> bool;
 
+    /// Returns the assigned <slot> element this element is assigned to.
+    ///
+    /// Necessary for the `::slotted` pseudo-class.
+    fn assigned_slot(&self) -> Option<Self> {
+        None
+    }
+
     fn has_id(&self,
               id: &<Self::Impl as SelectorImpl>::Identifier,
               case_sensitivity: CaseSensitivity)