automata: remove some unsafe code

[joshlf]

Note that the changes in wire.rs require zerocopy's "derive" feature, but the changes in accel.rs do not. If you'd prefer not to take a dependency on a proc macro derive, we can still salvage some of this PR.

[BurntSushi]

I was aware of zerocopy (and similar crates) when I wrote this code, and knew that it could probably eliminate some unsafe blocks. But as a rule of thumb, I don't add dependencies to regex. I have spent a lot of time and effort to keep its dependency tree small and reasonable (the entire tree is owned by me for example). So not only does taking a proc macro dependency not meet my bar, zerocopy itself doesn't either. Or at least, it would need to provide a much bigger benefit.

The way to get rid of these unsafe blocks is to push on the "safe transmute" effort I think.

[joshlf]

Yeah no worries, I understand the desire to avoid dependencies. We do the same. Happy to just close this if you'd prefer.

As for safe transmute, IIUC (@jswrenn is more directly involved and can provide more context), the idea is that safe transmute will basically just enable a trait bound, and you'll still need library code to provide useful abstractions given that bound. Roughly, safe transmute will replace zerocopy-derive, but not zerocopy (and presumably the same for bytemuck/bytemuck-derive, etc), and code like this PR would still require zerocopy in order to avoid writing unsafe.

[BurntSushi]

Aye. Understood. Thanks for the attempt! I do overall support trying to remove unsafe blocks, but probably not with the addition of other dependencies.