moves GroupProducer into impl { }
moves GroupProducer into impl { }

ryanhaining/cppitertools,tempbottle/cppitertools,jkhoogland/cppitertools,jkhoogland/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools