introduce BiFunction<A, B, V> as a direct alternative to Function<Tuple2<A,B>,V>
Add Functions.fromPair(PairFunction)
Add Functions.fromFunction(Function)
change reduce to scan + last, remove unused supplier alternative and batchSize as window+flatMap does exactly offer a solution
make sure scan is now using BiFunction
scan now properly gives as a first argument the N-1 value and the second argument the N value
+ Stream#last() to take the last item and emit it before complete