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
I think that it wasn't necessary to use list comprehensions on this one, just the `find` would be sufficient.. you are basically building the same list.

`[0..]` is an infinity generator, since it is lazy, you don't have to care its size