Fix worker/wrapper ids for newtype instances

Fix worker/wrapper ids for newtype instances
- mkDataConIds forced an algebraic worker/wrapper on newtype instances (not
  on vanilla newtypes).  I am not sure where this came from (from code I
  wrote or from the recent restructuring of MkIds), but its not the right
  thing to do.  In particular, it requires newtype instances to have con_info
  labels in the generated code, which makes no sense.
- Worker construction for newtypes is smart enough that it will construct the
  right signature for newtype instances (ie, one that mentions the family
  type constructor, not the instance tycon).