iter 1.3

• Added Iterator iter\product(...$iterables):
  Returns the cartesian product iterator of the passed iterators. (#25)
• Added bool iter\isIterable(mixed $value):
  Returns whether the value is an array or Traversable.
• Added $key parameter to the callback of reduce() and reductions(). (#28)
• Added support for <=> to fn\operator(). It is also supported on PHP versions that do not natively provide a <=> operator.
• All functions now validate that inputs are iterables.