Changes in the signature of some methods of coproduct

Methods `split`, `splitC`, `rotateLeft`, and `rotateRight` each have 2 signatures, but one of them cannot be called, like in https://github.com/milessabin/shapeless/pull/209. This PR fixes this the same way as in https://github.com/milessabin/shapeless/pull/209 (and https://github.com/milessabin/shapeless/pull/172).