Tracking issue for zerocopy

[KodrAus]

This issue is a place to keep anybody using the unstable zerocopy feature up-to-date on any changes being made to it.

In order to use zerocopy, you need to opt-in not just to the feature, but to the uuid_unstable RUSTFLAGS configuration too. We can make breaking changes to zerocopy at any point, but still want to be considerate of anybody making use of it, so will try keep open about any version bumps or other breakage that's coming.