Rollup merge of #107429 - tgross35:from-bytes-until-null-stabilization, r=dtolnay

Stabilize feature `cstr_from_bytes_until_nul`

This PR seeks to stabilize `cstr_from_bytes_until_nul`.

Partially addresses #95027

This function has only been on nightly for about 10 months, but I think it is simple enough that there isn't harm discussing stabilization. It has also had at least a handful of mentions on both the user forum and the discord, so it seems like it's already in use or at least known.

This needs FCP still.

Comment on potential discussion points:
- eventual conversion of `CStr` to be a single thin pointer: this function will still be useful to provide a safe way to create a `CStr` after this change.
- should this return a length too, to address concerns about the `CStr` change? I don't see it as being particularly useful, and it seems less ergonomic (i.e. returning `Result<(&CStr, usize), FromBytesUntilNulError>`). I think users that also need this length without the additional `strlen` call are likely better off using a combination of other methods, but this is up for discussion
- `CString::from_vec_until_nul`: this is also useful, but it doesn't even have a nightly implementation merged yet. I propose feature gating that separately, as opposed to blocking this `CStr` implementation on that

Possible alternatives:

A user can use `from_bytes_with_nul` on a slice up to `my_slice[..my_slice.iter().find(|c| c == 0).unwrap()]`. However; that is significantly less ergonomic, and is a bit more work for the compiler to optimize compared the direct `memchr` call that this wraps.

## New stable API

```rs
// both in core::ffi

pub struct FromBytesUntilNulError(());

impl CStr {
    pub const fn from_bytes_until_nul(
        bytes: &[u8]
    ) -> Result<&CStr, FromBytesUntilNulError>
}
```

cc ```@ericseppanen``` original author, ```@Mark-Simulacrum``` original reviewer, ```@m-ou-se``` brought up some issues on the thin pointer CStr

```@rustbot``` modify labels: +T-libs-api +needs-fcp