I need functionality of map, but done in such a way that the output array
is given as the argument, not as a return value. The closest I could get was

let inplace_map f a b = Array.blit (map f a) 0 b 0 (Array.length a)

Arrays a and b are of the same size.
This seems very inelegant.

One could use an adapter function and iteri, but that adds very noticeable 
overhead, and doesn't seem too elegant either.

let inplace_map f a b = Array.iteri (fun i src -> b.(i) <- f src; ()) a

There must be a better way of doing it, right? The given here is that the 
arrays a and b are there to stay, so we have to use them in-place.

Cheers, Kuba