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)

lib/parfait/type.rb

@@ -190,6 +190,7 @@ module Parfait
     def add_instance_variable( name , type )
       raise "No nil name" unless name
       raise "No nil type" unless type
+      return self if @names.index_of(name)
       hash = to_hash
       hash[name] = type
       return Type.for_hash( hash , object_class)