Replace `Option` by `Opt`

Now that we have an unboxed option-like class, should we start to use it more extensively ?

The usage in internal code and private methods is uncontroversial, but changing the API could be.

In particular, if we change public methods to `Opt`, we must be careful that the `Opt` API is complete enough, or that an implicit conversion to `Option` takes care of all use cases.
