style: Make MatchingContext generic over SelectorImpl.

This will help Xidorn implement tree pseudos, and in general makes sense,
allowing to put specific matching data in a selectors implementation.

components/script/dom/element.rs

@@ -2528,11 +2528,11 @@ impl<'a> SelectorsElement for DomRoot<Element> {
         self.upcast::<Node>().GetParentElement()
     }
 
-    fn match_pseudo_element(&self,
-                            _pseudo: &PseudoElement,
-                            _context: &mut MatchingContext)
-                            -> bool
-    {
+    fn match_pseudo_element(
+        &self,
+        _pseudo: &PseudoElement,
+        _context: &mut MatchingContext<Self::Impl>,
+    ) -> bool {
         false
     }
 
@@ -2594,13 +2594,15 @@ impl<'a> SelectorsElement for DomRoot<Element> {
         self.namespace()
     }
 
-    fn match_non_ts_pseudo_class<F>(&self,
-                                    pseudo_class: &NonTSPseudoClass,
-                                    _: &mut MatchingContext,
-                                    _: &RelevantLinkStatus,
-                                    _: &mut F)
-                                    -> bool
-        where F: FnMut(&Self, ElementSelectorFlags),
+    fn match_non_ts_pseudo_class<F>(
+        &self,
+        pseudo_class: &NonTSPseudoClass,
+        _: &mut MatchingContext<Self::Impl>,
+        _: &RelevantLinkStatus,
+        _: &mut F,
+    ) -> bool
+    where
+        F: FnMut(&Self, ElementSelectorFlags),
     {
         match *pseudo_class {
             // https://github.com/servo/servo/issues/8718