Update reference safety requirements
Per https://github.com/rust-lang/rust/pull/116677#issuecomment-1945495786, the language as written promises too much. This PR relaxes the language to be consistent with current semantics. If and when #117945 is implemented, we can revert to the old language.
While we're here, we also require that references be non-null.
cc ``@RalfJung``