Add a varient of unfoldrN

[tommd]

I suggest we either add a variant of unfoldrN or make an API-breaking change to the current one. The variant (placeholder name for now) would have a signature:

unfoldrN'

Storable a => Int -> (b -> Maybe (a, b)) -> b -> (Vector a,b)

In otherwords, we provide the resulting type b after applying the function N time (or fewer, if it returns Nothing).