Hoist style-only traversal into geckolib.

Implementing StandaloneStyleContext::new is going to require a TLS key (just
like LayoutContext::new), and we don't want that key in shared code.

ports/geckolib/lib.rs

@@ -25,4 +25,5 @@ extern crate util;
 mod bindings;
 #[allow(non_snake_case)]
 pub mod glue;
+mod traversal;
 mod wrapper;