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/slot.rb

@@ -42,13 +42,13 @@ module Virtual
   end
 
   class FrameSlot < Slot
-    def initialize index , type = Mystery, value = nil
+    def initialize index , type = Unknown, value = nil
       super
     end
   end
 
   class SelfSlot < Slot
-    def initialize index , type = Mystery, value = nil
+    def initialize index , type = Unknown, value = nil
       super
     end
   end