Simplify `FromPyPointer`

[davidhewitt]

There are so many methods in this trait. Is there a reason why we don't just have

unsafe fn from_owned_ptr(py: Python<'p>, ptr: *mut ffi::PyObject) -> PyResult<&'p Self>;
unsafe fn from_borrowed_ptr(py: Python<'p>, ptr: *mut ffi::PyObject) -> PyResult<&'p Self>;

As the only trait method. And then just let users choose if they want to .ok(), .unwrap(), or .expect() this, instead of all the _or_opt, _or_panic etc.

Originally posted by @davidhewitt in #770

[kngwyu]

We use a special panic implementation that calls ffi::PyErr_Print so I'd rather remove only from_owned_ptr_or_opt.

[davidhewitt]

Ok. I think after #492 is addressed we will probably find that the custom panic implementation is not necessary. So let's wait with this until #492 is done.

[davidhewitt]

FromPyPointer has been removed with the GIL Refs.