[WIP] Add PyTypeObject impl to PyRef

[konstin]

As @DeflatedPickle noticed on gitter, there was no way to get the PyType from the associated type of a PyRef.

Open question: Do we want to add the following?

impl<T> AsPyRef<T> for T
where
    T: PyTypeInfo,
{
    #[inline]
    fn as_ref(&self, _py: Python) -> PyRef<T> {
        PyRef::from_ref(self)
    }
    #[inline]
    fn as_mut(&mut self, _py: Python) -> PyRefMut<T> {
        PyRefMut::from_mut(self)
    }
}

[kngwyu]

I'm not for this change.
PyRef exists because we cannot always get *mut ffi::PyObject from &T whre T: PyTypeInfo.
But this change allows pyclass to use as_ptr without copying it to Python heap

[kngwyu]

Open question: Do we want to add the following?

I strongly disagree. It's unsafe and misleading.

I think get_type_from_object is OK but still can't imagine its concrete use cases.
So I think it's better to fix #381 first, and check the problem still remains or not.

[konstin]

I need to rethink this with a proper approach