Fix PyLayout and restrict PyRef::as_ref for non-native types

[kngwyu]

Fix #782.
Simply I make get_ptr a private method of PyCellInner.
This change forbids to get a base object of #[pyclass(extends=NativeType] style pyclasses.
Current &PyAny style API uses mainly a pointer of a pointer as an object handle, and integrates really badly with new PyCell 😣

[kngwyu]

FYI, this is a valid implementation of get_ptr for PyAny

unsafe fn get_ptr(&self) -> *mut PyAny {
    self.py().from_borrowed_ptr(self as *const _ as *mut _) as *const PyAny as *mut _
}

... but I'm not sure we should do so.
It would work but I'm sure it's a dirty solution in that it stores the same pointer 2 times.

[davidhewitt]

I've had a read through and looks fine. It's a shame to lose the safe API to access the super class.

I want to think for a bit longer and see if I can figure out another way to solve this.

[kngwyu]

It's a shame to lose the safe API to access the super class.

Yeah, but since native type inheritance was broken in PyO3 < 0.9, it's a not breaking change and not very problematic.

[davidhewitt]

Agreed. I suggest we merge this PR to fix the safety issue, and I'll create a separate issue for finding a safe API for upcasting.