Support zerocopy

[qwandor]

As per #310, for users of bitflags to continue being able to derive zerocopy traits on bitflags-generated types, it appears that there will need to be a cargo feature flag to make the internal bitflags types derive the relevant zerocopy traits.

[KodrAus]

We should come up with a strategy for supporting these unstable libraries. I've opened #329 to come up with one.

[KodrAus]

Having spent some time trying to make zerocopy supportable in bitflags I've come to the conclusion that we won't be able to directly support any libraries that require custom derives.

I am working on a general solution to this though so that you can partially "eject" your flags definition and can add derives for libraries bitflags doesn't directly support without losing all the nice codegen. It currently looks like this.

[qwandor]

Thanks for looking into this!

[KodrAus]

Thanks for checking this out @qwandor 👍

As per #326 (comment), we can now work with zerocopy without needing to support it directly in bitflags.