New lints: repr(packed) added or removed on a struct or union

[obi1kenobi]

Both adding and removing repr(packed) is a breaking change when it happens on either structs or unions:
https://doc.rust-lang.org/cargo/reference/semver.html#repr-packed-add
https://doc.rust-lang.org/cargo/reference/semver.html#repr-packed-remove

Adding and removing are definitely going to need separate lints. Whether structs and unions each have their own lints (so, total of 4 lints across all combinations) or we combine them together (so, 2 lints total: repr(packed) added, repr(packed) removed) is up to the implementer.