modulize return and list

lib/typed/tree/return_statement.rb

@@ -1,5 +1,7 @@
 module Typed
-  class ReturnStatement < Statement
-    attr_accessor :return_value
+  module Tree
+    class ReturnStatement < Statement
+      attr_accessor :return_value
+    end
   end
 end

lib/typed/tree/to_code.rb

@@ -89,7 +89,7 @@ module Typed
     alias :on_false_statements :on_statements
 
     def on_return statement
-      w = ReturnStatement.new()
+      w = Tree::ReturnStatement.new()
       w.return_value = process(statement.children.first)
       w
     end