Restore support for &Class and &mut Class as arguments

[davidhewitt]

We can't implement FromPyObject for them. You can use PyRef and PyRefMut as arguments.

Agreed we can't implement FromPyObject for them. It would be nice if we can find another way for the generated wrappers to support &Class and &mut Class arguments - by calling .try_borrow() in the wrapper, perhaps, like we can do for &self.

Originally posted by @davidhewitt in #785 (comment)

[kngwyu]

I'm experimenting an idea to change our generated method call(_slf.#fname(#name1, #name2, ...) to _slf.#fname(&*#name1, &*#name2, ...) only when an arg(e.g., #name1) is reference type.
Though I'm not sure it's possible 🤔