Fix Trac #3966: warn about useless UNPACK pragmas
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.
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