This completes the support for generic programming introduced
in GHC 7.2. Generic1 allows defining generic functions that
operate on type containers, such as `fmap`, for instance.

Along the way we have fixed #5936 and #5939, allowing
deriving Generic/Generic1 for data families, and disallowing
deriving Generic/Generic1 for instantiated types.

Most of this patch is Nicolas Frisby's work.