Traits should be clear about stability guarantees regarding UnsafeCell

[joshlf]

Currently, FromZeroes, FromBytes, and AsBytes require that implementing types contain no UnsafeCells. However, we may lift that restriction in the future.

If any code takes our ban on UnsafeCells to mean that they can use, e.g. T: FromZeroes as a guarantee that T contains no UnsafeCells, then if we were to relax this requirement, we could allow their code to become silently unsound.

We should update our docs to be very precise and very loud about what external code may take as stability guarantees. We should also guide users towards the zerocopy API itself and discourage using our traits to gate user-written unsafe code.