Add solutions for chapter 5.

@fabioyamate, @laurocaetano I'm so sleepy that I don't remember where I had to change on the last exercise, haha.
I think this is a case where the `()` is required

it could be rewritten to:

```haskell
head $ positions (minimum chitab) chitab
```

however the `minimum` requires the `()`, so pointfree notation is not possible.