style: Add code to make part rules affect the style of the elements.

I still haven't implemented each_part(), so this will do nothing yet.

The cascade order stuff is fishy, I know, and I'll fix in a followup if it's
fine with you. I moved the sorting of the rules to rule_collector, since it
seemed to me it was better that way that duplicating the code, and those
SelectorMap functions only have a single caller anyway.

Differential Revision: https://phabricator.services.mozilla.com/D32647

components/style/dom.rs

@@ -520,6 +520,9 @@ pub trait TElement:
     /// Whether this element has an attribute with a given namespace.
     fn has_attr(&self, namespace: &Namespace, attr: &LocalName) -> bool;
 
+    /// Returns whether this element has a `part` attribute.
+    fn has_part_attr(&self) -> bool;
+
     /// The ID for this element.
     fn id(&self) -> Option<&WeakAtom>;
 
@@ -528,6 +531,13 @@ pub trait TElement:
     where
         F: FnMut(&Atom);
 
+    /// Internal iterator for the part names of this element.
+    fn each_part<F>(&self, _callback: F)
+    where
+        F: FnMut(&Atom)
+    {
+    }
+
     /// Whether a given element may generate a pseudo-element.
     ///
     /// This is useful to avoid computing, for example, pseudo styles for