Compile into versioned subfolder of .curry

Since the different compiler implementations (KiCS2, PAKCS, ...) use slightly different FlatCurry representations, especially among different versions, the compilation artifacts (FlatCurry, generated Haskell code) should be placed in a versioned subfolder of .curry to avoid conflicts when switching compiler versions. They could, for example, be placed in

.curry/pakcs-x.y.z

where x.y.z is the version number of the PAKCS compiler.