implement toList.

hs/DataStructures.hs

fromList [] = nil
fromList (x:xs) = cons x $ fromList xs
toList :: (Seq s) => s a -> [a]
toList s | isEmpty s = []
toList s =
    case first s of
        Just x -> x : toList (rest s)
butLast :: (Seq s) => s a -> s a
butLast s | isEmpty s = nil
butLast s =