Generate warning-free Haskell code

[andreasabel]

Recover the property of BNFC 2.7.1 to generate warning-free Haskell code
https://github.com/BNFC/bnfc/blame/0ad87322fce4cfe309512fa9fd56a63a06ff9c19/source/CHANGELOG.md#L117

• Abs.hs: unused imports: fixed in a351053
• Abs.hs: no type signatures for defined constructors with --functor: fixed in 195cc1b
• Layout.hs: unused-matches: fixed in 4343b31
• Print.hs: locals (like id) shadow globals (see also Haskell: local variables in Printer may clash with printing functions #337): fixed in 3b2029e
• Skel.hs: has unused-matches, need to turn warning off: fixed in 89e5914
• Test.hs: misc: fixed in 5208f67
• XML.hs
• XMLT.hs
• Haskell/GADT