Incomplete features can also be unsound
Some incomplete features do not just ICE, they are also currently unsound (e.g. https://github.com/rust-lang/rust/pull/72029, and also `specialization` -- which is not yet marked incomplete but [should be](https://github.com/rust-lang/rust/pull/71420)). This makes the message reflect that.
While at it I also added a link to the tracking issue, which hopefully should explain what is incomplete/unsound about the feature.
- uses a never-stable core::array::LengthAtMost32 to bound the impls
- includes a custom error message to avoid mentioning LengthAtMost32 too often
- doesn't use macros for the slice implementations to avoid #62433
