built-in derive: remove BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE hack and lint Fixes https://github.com/rust-lang/rust/issues/107457 by turning the lint into a hard error. The lint has been shown in future breakage reports since Rust 1.69 (released in April 2023). Let's see (via crater) if enough time has passed since https://github.com/rust-lang/rust/pull/104429, and https://github.com/unicode-org/icu4x/pull/2834 has propagated far enough to let us make this a hard error. Cc ``@nnethercote`` ``@Manishearth``