Opaque PyObject ABI support

[ngoldbaum]

Towards fixing #5786.

This is done on top of #5753 because that PR is needed to make this work.

With some help from @davidhewitt, this almost works but subclassing seems to be broken.

---- pycell::tests::test_pyref_as_super stdout ----
[src/pycell.rs:833:13] &pyref.inner = <builtins.SubSubClass object at 0x104ee6940>
[src/pycell.rs:834:13] &pyref.as_super().inner = <builtins.SubSubClass object at 0x104ee6940>

thread 'pycell::tests::test_pyref_as_super' (263860852) panicked at src/pycell.rs:835:13:
assertion `left == right` failed
  left: 20
 right: 10

I suspect using Layout to fill in LayoutAsBase is incorrect here:

pyo3/pyo3-macros-backend/src/pyclass.rs

Line 2833 in 5e317f0

type LayoutAsBase = <Self as #pyo3_path::impl_::pyclass::PyClassImpl>::Layout;

But I'm not sure how to update the macros and the macro wrappers to get that to work correctly.

@Icxolu do you happen to have any insight about what's going wrong?

TODO:

• Hide PyModuldeDef in the FFI bindings.
• Make tests fully pass

[ngoldbaum]

Interesting: the careful tests fail in exactly the same way as the tests fail on the opaque pyobject build. Maybe a useful hint that there's some unsafety happening somewhere...