Replace all uses of bytemuck with zerocopy

[TechnoPorg]

Closes #1200. Unwraps have been used where bytemuck would panic on a size mismatch, or where the size is explicitly checked before performing the conversion.

[davidlattimore]

Thanks! One of the failures looks unrelated to your change and likely to be a flake. Hopefully once you run cargo fmt and reupload, the flake won't happen. Not sure how the flake is possible since we're supposed to have file locking to prevent that kind of thing