Migrate to `safer_owning_ref`

[noamtashma]

After the maintainer of owning_ref has been unresponsive for a long time, I decided to publish my fix of owning_ref known soundness issues as a new crate, safer_owning_ref.

Please migrate to safer_owning_ref.

[RazrFalcon]

It's used only in tests, so it's fine for now.