Add support for custom printers in views.

Let's call the new parameter `format`, for consistency with [Ctypes.format][ctypes-format] and [Ctypes.format_typ][ctypes-format-typ].

[ctypes-format]: https://github.com/ocamllabs/ocaml-ctypes/blob/297e9c93/src/ctypes/ctypes.mli#L422
[ctypes-format-typ]: https://github.com/ocamllabs/ocaml-ctypes/blob/297e9c93/src/ctypes/ctypes.mli#L407