Provoked by questions from Johan

 - Improve comments, fix misleading stuff
 - Add commented synonyms for HsSrcBang, HsImplBang, and use them throughout
 - Rename HsUserBang to HsSrcBang
 - Rename dataConStrictMarks to dataConSrcBangs
          dataConRepBangs    to dataConImplBangs

This renaming affects Haddock in a trivial way, hence submodule update