Rewrite Freevars computation and remove generic visitor

[BinderDavid]

This is almost ready. I am still missing the correct logic for this place:

impl FV for ust::Case {
    fn visit_fv(&self, _v: &mut USTVisitor) {
        let ust::Case { info: _, name: _, args: _, body: _ } = self;

        //body.visit_fv(v);
        // TODO: The visitor context must be extended somehow.
    }
}

The tests are running through, but this is not a good sign. We don't have any tests which check xfunc at the moment.