Simplifying HList.fill and Tuple.fill definitions

Following the discussion in https://github.com/milessabin/shapeless/pull/172, the signatures of HList.fill and Tuple.fill evolved and the FillBuilder classes which were used in the first proposed implementations are no longer required. This PR drops them. This moves the type argument of the filling element to a more conventional place (see the modified test lines).
See my comment in https://github.com/milessabin/shapeless/pull/185 about `testFill`. This method indeed tests that.