Use ciborium impl of Value::into_

[dodomorandi]

Once this PR is landed, we can remove our extension trait and directly use Value::into_ methods.

Blocked by enarx/ciborium#34

[lmammino]

I just realised that enarx/ciborium#34 was merged, so we can probably get this one done :)

[lmammino]

Ok, i take that back. It seems those changes are not published yet, so we need to wait for this new issue: enarx/ciborium#37