Provide zerocopy trait implementations behind feature flag

[joshlf]

A common use of unsafe code is to access or modify the bytes of libc types (e.g. by transmuting them; here's an example). Many libc types could implement the zerocopy traits (FromBytes, AsBytes, and Unaligned) by using a custom derive, so this would just be a matter of adding #[cfg_attr(feature = "zerocopy", derive(FromBytes, AsBytes, Unaligned))] to the types which support it. This would allow all of those uses to be safe.

If this is something that the maintainers would be open to, I'd be happy to put up a PR.