On replacing UnsafeCell

Here are some pointers:

• repr(packed) allows invalid unaligned loads · Issue #27060 · rust-lang/rust · GitHub
• Tracking issue for safe_packed_borrows compatibility lint · Issue #46043 · rust-lang/rust · GitHub
• Accessing DST field of packed struct calculates the wrong field address · Issue #80925 · rust-lang/rust · GitHub
• Misaligned reference from drop field in packed struct · Issue #99838 · rust-lang/rust · GitHub
• Undetected unsound usage of packed field by unsize coercion · Issue #115396 · rust-lang/rust · GitHub
• We are computing the wrong offset for unsized packed structs · Issue #118537 · rust-lang/rust · GitHub

As you can tell from the sheer number of soundness issues, this has not been a role model we want to copy.