servo: Merge #18384 - style: Don't waste a whole selector map for each class / id in the document (from emilio:invalidation-map-bloat); r=bholley

On top of #18375, only last commit needs review.

Source-Repo: https://github.com/servo/servo
Source-Revision: 4721ef81fd7fb6ee15546a6abe140a66ce0280e8

servo/components/style/selector_parser.rs

@@ -188,4 +188,9 @@ impl<T> PerPseudoElementMap<T> {
         }
         Ok(self.entries[index].as_mut().unwrap())
     }
+
+    /// Get an iterator for the entries.
+    pub fn iter(&self) -> ::std::slice::Iter<Option<T>> {
+        self.entries.iter()
+    }
 }