Use Gecko's simpler Bloom filter instead of one based on hash

stretching.

This preserves the usage of the Bloom filter throughout style recalc,
but the implementation is rewritten. Provides a 15% improvement on
Guardians of the Galaxy.

components/style/node.rs

@@ -38,4 +38,5 @@ pub trait TElement<'a> : Copy {
     fn get_disabled_state(self) -> bool;
     fn get_enabled_state(self) -> bool;
     fn has_class(self, name: &str) -> bool;
+    fn each_class(self, callback: |&Atom|);
 }