changed ProloGraalBuiltinClauses into Truffle InstrumentableNodes, but builtins are still executed by java classic overriding.
changed ProloGraalIsBuiltin into specialized abstract class, now using ProloGraalIsBuiltinNodeGen.create to instantiate it (Truffle generated node).
created ProloGraalTypes class to allow new ProloGraal types into node specializations, currently WIP.