Colocate Traversal classes with classes they traverse

This puts all knowledge of the traversal in the same file, and removes
the need for a CASES registry.

lib/liquid/variable_lookup.rb

@@ -78,5 +78,11 @@ module Liquid
     def state
       [@name, @lookups, @command_flags]
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        @node.lookups
+      end
+    end
   end
 end