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/condition.rb

@@ -128,6 +128,15 @@ module Liquid
         end
       end
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        [
+          @node.left, @node.right,
+          @node.child_condition, @node.attachment
+        ].compact
+      end
+    end
   end
 
   class ElseCondition < Condition