problem).  inherit_slots() is split in two parts: inherit_special()
  which inherits the flags and a few very special members from the
  dominant base; inherit_slots() which inherits only regular slots,
  and is now called for each base in the MRO in turn.  These are now
  both void functions since they don't have error returns.

- Added object.__setitem__() back -- for the same reason as
  object.__new__(): a subclass of object should be able to call
  object.__new__().

- add_wrappers() was moved around to be closer to where it is used (it
  was defined together with add_methods() etc., but has nothing to do
  with these).