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.rb

@@ -138,5 +138,11 @@ module Liquid
         raise error
       end
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        [@node.name] + @node.filters.flatten
+      end
+    end
   end
 end