Should the version constraint be `>= "0.3"` (here and in the other opam files)?  [`CArray`](https://github.com/andrewray/iocaml/blob/v0.4.4/iocaml_zmq.ml#L242) is [new in 0.3](https://github.com/ocamllabs/ocaml-ctypes/releases/tag/ocaml-ctypes-0.3).