Exercise 5.6: Added proper flatMap implementation (i.e. given answer) for a Stream.
Exercise 5.7: Implemented the function constant.

Generalize ones slightly to the function constant which returns an infinite
Stream of a given value.
Exercise 5.8: Implemented Stream.from.

Write a function that generates an infinite stream of integers, starting from n,
then n + 1, n + 2, etc.
Exercise 5.9: Implemented Stream.fibs.

Write a function fibs that generates the infinite stream of Fibonacci
numbers: 0, 1, 1, 2, 3, 5, 8, and so on.