Warning about useless UNPACK pragmas wasn't as easy as I thought.
I did quite a bit of refactoring, which improved the code by refining
the types somewhat.  In particular notice that in DataCon, we have

    dcStrictMarks   :: [HsBang]
    dcRepStrictness :: [StrictnessMarks]

The former relates to the *source-code* annotation, the latter to
GHC's representation choice.