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.

Lib/test/test_descrtut.py

@@ -190,6 +190,7 @@ Instead, you can get the same information from the list type:
      '__contains__',
      '__delattr__',
      '__delitem__',
+     '__delslice__',
      '__eq__',
      '__ge__',
      '__getattribute__',