Chapter 7: Higher-order functions

@carlosgaldino and @laurocaetano Finally I finished the chapter 7 exercises.

The exercise 7 took me some time because it wasn't work as I expected. But, once I did the evaluation steps it  was easy to fix it.

The exercises 8 and 9, not sure if it was the best solution.
yeah, I misinterpreted the behavior of `takeWhile` and `dropWhile`. Going to fix that. This is what happens when you do 1am.