rename Mystery

just unknown, mystery should be more difficult to find out.
After all we keep run time info, so just need unknown at compile time

lib/virtual/slots/message_slot.rb

@@ -1,26 +1,26 @@
 module Virtual
   # The message that is being processed has a layout as per the constant above
   class MessageSlot < Slot
-    def initialize index , type = Mystery , value = nil
+    def initialize index , type = Unknown , value = nil
       super(index ,type , value )
     end
   end
 
   class Return < MessageSlot
-    def initialize type = Mystery, value = nil
+    def initialize type = Unknown, value = nil
       super( MESSAGE_RETURN_VALUE , type , value  )
     end
   end
 
   class Self < MessageSlot
-    def initialize type = Mystery, value = nil
+    def initialize type = Unknown, value = nil
       super( MESSAGE_SELF , type , value  )
     end
   end
 
   # Name of the current message
   class Name < MessageSlot
-    def initialize type = Mystery, value = nil
+    def initialize type = Unknown, value = nil
       super( MESSAGE_NAME , type , value  )
     end
   end