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 remember this problem when I was trying the past Google code jam https://code.google.com/codejam/contest/32016/dashboard#s=p0

at that time I didn't use list comprehensions