Chapter 5 - List comprehensions

These are the solutions for the fifth chapter. I had lot of fun solving these problems, especially the exercise number five.

PS.. I didn't like the solution for the eighth exercise, mainly the `let*2int` and `int2let*` functions. I tried to make just one function, but I had no success. I'll try later this week.

@fabioyamate @carlosgaldino
After looking at @fabioyamate PR, I have a feeling this exercise is incomplete. I'll update it.