…r=dtolnay

clarify that unsafe code must not rely on our safe traits

This adds a disclaimer to PartialEq, Eq, PartialOrd, Ord, Hash, Deref, DerefMut.

We already have a similar disclaimer in ExactSizeIterator (worded a bit differently):
```
/// Note that this trait is a safe trait and as such does *not* and *cannot*
/// guarantee that the returned length is correct. This means that `unsafe`
/// code **must not** rely on the correctness of [`Iterator::size_hint`]. The
/// unstable and unsafe [`TrustedLen`](super::marker::TrustedLen) trait gives
/// this additional guarantee.
```
If there are any other traits that should carry such a disclaimer, please let me know.

Fixes rust-lang#73682

…er-errors

make interpreter and TyAndLayout type Debug impl independent of Ty debug impl

This fixes some (but not all) of the fallout from rust-lang#115661.

Second commit is taken from rust-lang#107084 (and slightly adjusted); I preserved the original git author information.