Enhance operators with optional state

[andres-kovalev]

It would be nice to hold some state between createFn calls. For instance, to be able to generate fibonacci sequence:

const next = last => {
    if (last.length < 2) {
      last.push(1);
      return 1;
    }

    const item = last.shift() + last[0];
    last.push(item);
    return item;
};

// before
const f = () => {
  const last = [];

  return () => next(last);
}

const fibonacciSequence = create(f(), 10); // ok
const fibonacciGenerator = create(f()); // not ok, will work only once

// after
const fibonacciGenerator = create((index, state) => {
  if (!state.last) state.last = [];

  return next(state.last);
})