Add types for None, Ellipsis and NotImplemented

[davidhewitt]

          Rather than returning `PyObject` here it would probably be nicer if we returned `&PyEllipsis`, and created a new struct `types::PyEllipsis`. (Would avoid the need to increase a reference count here.)

That would also mean that x.is_instance::<PyEllipsis>() would just work instead of needing is_ellipsis, which does have the upside of avoiding a special method. Not necessarily saying that it's better, perhaps is_ellipsis is still warranted.

Also, adding types::PyEllipsis would pave the way for instead just having PyEllipsis::get(py) to retrieve the singleton, rather than adding this to Python which I don't think really needs this method.

Originally posted by @davidhewitt in #2911 (comment)