Haskell-backend: Do not generate trailing whitespace.

Removed trailing white space in Haskell source code strings, such that
the generated .hs files do not have trailing whitespace.
Haskell backend: top-level type signatures in generated code.

Missing top-level type signatures generate a warning.
Added all missing top-level sigs.
Haskell backend: _ for unused variables in generated code.

To make the generated Haskell code error free, use _ or _ident for
unused bindings everywhere.
Merge pull request #109 from andreasabel/master

Haskell backend: generate warning free code.