Fixed the shared slot object bug

All instances of the same concept used (read:  “overwrite”) the same
slot object.

As an overhead for fixing that, for each concept’s constructor you have
to call init_slots() so that the slots are re-instantiated for each
individual concept instance. (If you put all slots in the constructor
though, the class method class_slots() will break)
Feel free to come up with a cleaner fix, but ask me first - I’ll save
you time.