Implement class instance variables

as they are just the type of the meta_class, that was relatively simple.
I feel this is what oo is meant to be, instance variables and methods for the objects, and since classes are objects, for them too.
Class variables seem like a design mistake, weird scoping rules and no data hiding (left as an exercise to the reader)

test/parfait/type/test_type_api.rb

@@ -58,6 +58,10 @@ module Parfait
       assert_equal :Type , @type.type_at(0)
     end
 
+    def test_add_again
+      assert_equal @type.to_hash , @type.add_instance_variable(:type , :Type).to_hash
+    end
+
     def test_inspect_added
       type = test_add_name
       assert type.inspect.include?("boo") , type.inspect