simplify implementation of Py::clone_ref

[davidhewitt]

Just browsing source and felt that this alternative implementation is simpler for the optimizer to inline away. (Should ultimately reach the same machine code either way.)

[alex]

I'm ambivalent on this, if you think it's cleaner that's fine.

[davidhewitt]

Agreed it's very minor; I just feel that Py::from_borrowed_ptr (which itself recurses into Py::from_borrowed_ptr_or_opt and introduces a panic that will never be reached in this case) produces a lot more stuff which needs to be removed in dead code elimination.