Failing to build under newer version of Why3

With a recent update in Why3 (1.7.0), many functions that were supported by Opt were scrapped, seeing as most of them were already implemented in the Stdlib of OCaml by the [Option](https://v2.ocaml.org/api/Option.html) module, and stale usages of Opt were replaced by Option. [See merge](https://gitlab.inria.fr/why3/why3/-/merge_requests/986) The current version of Cameleer still uses Why3's Opt and since there is no upper bound on the Why3 dependency, it is unable to build under Why3 1.7.0. I believe that reflecting the same changes and switching over to Option fixes the failure to build.