The new type system allows non-empty __slots__ only
for fixed-size objects.

Meta-types are types which instances are also types.
types are variable-sized, because they take the slot
definitions for their instances, so the cannot have
extra members from their meta-type.

The proposed solution allows for two things:
a) meta-types can have slots
b) extensions get access to the whole type object and
    can create extended types with private fields.

The changes providing this are quite simple:
- replace the internal hidden "etype" and turn it into
  an explicit PyHeapTypeObject in object.h
- instead of a fixed offset into the former etype, the
slots
  calculation is based upon tp_basicsize.

To keep things easy, I added a macro which does this
calculation, and member access read now like so:

before:
	type->tp_members = et->members;
after:
	type->tp_members = PyHeapType_GET_MEMBERS(et);

This patch has been tested thoroughly in my own code since
Python 2.2, and I think it is ripe to get into the
distribution.
It has almost no impact on speed or complexity.