Halfway checkin. This is still messy, but it's beginning to address

Halfway checkin.  This is still messy, but it's beginning to address
the problem that slots weren't inherited properly.  override_slots()
no longer exists; in its place comes fixup_slot_dispatchers() which
does more and different work and is table-based.  (Eventually I want
this table also to replace all the little tab_foo tables.)

Also add a wrapper for __delslice__; this required a change in
test_descrtut.py.