This fixes most of Trac #3990.  Consider
  data family D a
  data instance D Double = CD Int Int
  data T = T {-# UNPACK #-} !(D Double)
Then we want the (D Double unpacked).

To do this we need to construct a suitable coercion, and it's much
safer to record that coercion in the interface file, lest the in-scope
instances differ somehow.  That in turn means elaborating the HsBang
type to include a coercion.

To do that I moved HsBang from BasicTypes to DataCon, which caused
quite a few minor knock-on changes.

Interface-file format has changed!

Still to do: need to do knot-tying to allow instances to take effect
within the same module.