NilType is now the base object/prototype that Nil is an instance of,
instead of Nil being a direct instance of Object. This allows us to
implement traits and redefine methods on the type of Nil (= NilType),
instead of Nil itself. This in turn allows the compiler to use a more
efficient way of representing types versus instances.

In type signatures you can still use Nil (or NilType, though Nil is
preferred), as this signals that you expect or return something
compatible with Nil, which is only Nil itself.