Add support for bytemuck

[KodrAus]

Closes #311

I've also added some docs on how to support external libraries here for future contributors. Since the bytemuck impls themselves are unsafe, I've added a note to where we generate our internal flags type warning that its ABI can't be changed.

[KodrAus]

cc @MarijnS95 do these look right to you? I haven't used bytemuck myself.

[KodrAus]

These have moved into the external module alongside their supporting code

[KodrAus]

I was getting a build error with Self being unknown here. Not sure if it was bytemuck's derives or a change in rustc but it was easily fixed.

[KodrAus]

cc @Lokathor 👋

I just wanted to make sure I've got these Pod and Zeroable impls right 🙂 In bitflags, we can automatically implement them on an internal type that looks like:

#[repr(transparent)]
struct FlagsField<T> {
    bits: T
}

where T may be a numeric type, like i32, usize etc. We may allow you to specify this type as something you've invented at some point in the future though. The type will always be a #[repr(transparent)] struct with a single field though.

The impls I've come up with look like this. Does that look right to you? Is there anything else we need to guarantee to make sure these impls are safe to do?

[Lokathor]

yep, impl<T> Pod for ReprTransparentIntNewtype<T> where T: Pod is basically exactly what I was expecting to see before I clicked, and then it's what I saw.

[KodrAus]

Awesome, thanks for checking them out! 🙇 I'll get this rolling through now.