Rollup merge of #60443 - RalfJung:as_ptr, r=SimonSapin

Rollup merge of #60443 - RalfJung:as_ptr, r=SimonSapin

as_ptr returns a read-only pointer

Add comments to `as_ptr` methods to warn that these are read-only pointers, and writing to them is UB.

[It was pointed out](https://internals.rust-lang.org/t/as-ptr-vs-as-mut-ptr/9940) that `CStr` does not even have an `as_mut_ptr`. I originally was going to add one, but there is no method at all that would mutate a `CStr`. Was that a deliberate choice or should I add an `as_mut_ptr` (similar to [what I did for `str`](https://github.com/rust-lang/rust/pull/58200))?