Draft: Fuzzer for BucketArray.

Hi @beth! This isn't ready as-is, and it doesn't even compile, but it might be a good starting point for a fuzzer on the unsafe logic in equix. I need to step away from this for now since it isn't really what I'm supposed to be working on right now.

In order to have this work, you'll need at a minimum to make the relevant parts of equix conditionally public when built with #[cfg(fuzzing)]. This can be done with a mixture of cfg_if, visible, and visibility. (Yes, both the visible and visibility crates seem to be necessary under different circumstances.)

I hope this is helpful and enjoyable; sorry about the draft quality.