layout: Implement ordered lists, CSS counters, and quotes per CSS 2.1

§ 12.3-12.5.

Only simple alphabetic and numeric counter styles are supported. (This
is most of them though.)

Although this PR adds a sequential pass to layout, I verified that on
pages that contain a reasonable number of ordered lists (Reddit
`/r/rust`), the time spent in generated content resolution is dwarfed by
the time spent in the parallelizable parts of layout. So I don't expect
this to negatively affect our parallelism expect perhaps in pathological
cases.

components/layout/table_colgroup.rs

@@ -104,6 +104,8 @@ impl Flow for TableColGroupFlow {
     fn iterate_through_fragment_border_boxes(&self,
                                              _: &mut FragmentBorderBoxIterator,
                                              _: &Point2D<Au>) {}
+
+    fn mutate_fragments(&mut self, _: &mut FnMut(&mut Fragment)) {}
 }
 
 impl fmt::Debug for TableColGroupFlow {