Use breaking version bump for fluent

* Closes #384 * Closes #385 * See also comments in #383. It seems we have an issue with downstream projects using (indirectly) `fluent-bundle::*` while at the same time the upstream project is using `fluent::fluent_bundle::*`. This is kind of a weird arrangement, but given that we're exposing it it does seem like the resolver has trouble getting the right combination. Note `cargo semver-checks` did *not* catch this, but that may be a bug. If we yank 0.16.2 and replace it with 0.17.0 we might mitigate some of the pain downstream. Thoughts?