Allow family instances in an hs-boot file
Compiling an .hs-boot file with a data instance leads to this error:
Illegal family instance in hs-boot file
Yet, data instances make good sense in an hs-boot file, in case another module wants to access a constructor declared in the instance.
Furthermore, it may sometimes be desirable to allow type family instances in a hs-boot file, as well, in case some including modules need the instance for type simplification.