layout: Implement basic lists and the CSS1 list properties.

The exact rendering is ill-spec'd. Some things are ugly (especially the
width and height of list style images) but they are infrequently used
and I believe this implementation matches the spec. Numeric lists are
not supported yet, since they will require a separate layout pass.

The implementation is a subclass of `BlockFlow`, on advice from Robert
O'Callahan.

components/layout/lib.rs

@@ -52,6 +52,7 @@ pub mod flow_ref;
 pub mod fragment;
 pub mod layout_task;
 pub mod inline;
+pub mod list_item;
 pub mod model;
 pub mod parallel;
 pub mod sequential;