Update "flatten" to be more versatile

[theMagicalKarp]

What

Update the flatten function to accept more than one iterable at a time, enhancing its versatility. This improvement enhances overall usability while ensuring backward compatibility.

items := itertools.Flatten(
	iter.New(
		iter.New(1, 2, 3),
		iter.New(4, 5, 6),
	),
	iter.New(
		iter.New(7, 8, 9),
	),
)
itertools.Print(items)
// Output:
// 1, 2, 3, 4, 5, 6, 7, 8, 9