fold_while implementation for CCList and CCArray (fixes #29)

I was looking for existing tests on these modules but couldn't find any. The tests I wanted to add were something like this:

```OCaml
(* Should return 2 *)
CCList.fold_while (fun acc b -> if b then acc+1, `Continue else acc, `Stop) 0 [true;true;false;true]

(* Should return 2 *)
CCArray.fold_while (fun acc b -> if b then acc+1, `Continue else acc, `Stop) 0 (Array.of_list [true;true;false;true])                                                                                                                      
```