From #310

Add an rkyv unstable Cargo feature that derives #[rkyv::{Serialize, Deserialize}] on the inner flags type so that end-users can then derive rkyv traits on the public one.

Since rkyv is still unstable, we should come up with some scheme that lets us continue to upgrade the dependency while it remains unstable without needing to bump the major version of bitflags itself. It wouldn't be such a big deal to bump bitflags if we don't make breaking changes to the BitFlags trait, since we can re-export its public API, but should consider other options first so that we don't end up needing to bump it too regularly for unstable dependencies.