Chapter 7 - HOFs

/cc @fabioyamate @laurocaetano
Well, if you look at the type definition it is what you expected.

```haskell
*Chapter7> :t unfold
unfold :: (t -> Bool) -> (t -> a) -> (t -> t) -> t -> [a]
```

But it takes time to get used to it and to remember that such function is available once you define it.