example use of Stream.unfold to generate a Fibonacci series.exs

iex> {1, 1} |> Stream.unfold(fn {a, b} -> {a, {b, a+b}} end) |> Enum.take(50)

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584,
 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229,
 832040, 1346269, 2178309, 3524578, 5702887, 9227465, 14930352, 24157817,
 39088169, 63245986, 102334155, 165580141, 267914296, 433494437, 701408733,
 1134903170, 1836311903, 2971215073, 4807526976, 7778742049, 12586269025]