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/style/stylist.rs

@@ -1198,7 +1198,7 @@ impl Stylist {
         animation_rules: AnimationRules,
         rule_inclusion: RuleInclusion,
         applicable_declarations: &mut V,
-        context: &mut MatchingContext,
+        context: &mut MatchingContext<E::Impl>,
         flags_setter: &mut F,
     )
     where