Accepted risk: `Immutable` types assumed not to contain `UnsafeCell`s

As part of #1155, we need guarantees that certain standard library types such as `Box` and `NonZeroXxx` and even built-ins such as `usize` do not contain `UnsafeCell`s. Per https://github.com/rust-lang/unsafe-code-guidelines/issues/492, it is difficult to get this guarantee, but it's also incredibly obvious that it's true and would be absurd to change (and certainly absurd in a way that doesn't just result in these types containing `UnsafeCell`s, but in a way that permits interior mutation to be exercised, which is really what `Immutable` cares about).

This issue tracks that this is an accepted risk, and we're going to proceed with 0.8 anyway.