Haskell backend: generate warning free code.

These three patches are minor improvements of the Haskell backend.  The BNFC-generated Haskell code should now no longer contain:
* trailing whitespace (generates red blobs in my emacs)
* top-level functions without signature (generates warning in GHC)
* unused variable bindings not starting with _ (generates warning in GHC)
[Sorry for the double negation...:-)]