Add `chunksOf`

What do you think about adding the following function?

```haskell
chunksOf :: Int -> Slist a -> Slist (Slist a)
```

For ordinary lists it would work like this:

```haskell
ghci> chunksOf 3 [0..7]
[[0,1,2], [3,4,5], [6,7]]
```

But since we have access to the list length, we can implement a more efficient version of `chunksOf`.