Failing to build under newer version of Why3

[ionchirica]

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 module, and stale usages of Opt were replaced by Option. See merge

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.