Rollup merge of #94984 - ericseppanen:cstr_from_bytes, r=Mark-Simulacrum

add `CStr` method that accepts any slice containing a nul-terminated string

I haven't created an issue (tracking or otherwise) for this yet; apologies if my approach isn't correct. This is my first code contribution.

This change adds a member fn that converts a slice into a `CStr`; it is intended to be safer than `from_ptr` (which is unsafe and may read out of bounds), and more useful than `from_bytes_with_nul` (which requires that the caller already know where the nul byte is).

The reason I find this useful is for situations like this:
```rust
let mut buffer = [0u8; 32];
unsafe {
    some_c_function(buffer.as_mut_ptr(), buffer.len());
}
let result = CStr::from_bytes_with_nul(&buffer).unwrap();
```

This code above returns an error with `kind = InteriorNul`, because `from_bytes_with_nul` expects that the caller has passed in a slice with the NUL byte at the end of the slice. But if I just got back a nul-terminated string from some FFI function, I probably don't know where the NUL byte is.

I would wish for a `CStr` constructor with the following properties:
- Accept `&[u8]` as input
- Scan for the first NUL byte and return the `CStr` that spans the correct sub-slice (see [future note below](https://github.com/rust-lang/rust/pull/94984#issuecomment-1070754281)).
- Return an error if no NUL byte is found within the input slice

I asked on [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/CStr.20from.20.26.5Bu8.5D.20without.20knowing.20the.20NUL.20location.3F) whether this sounded like a good idea, and got a couple of positive-sounding responses from ``@joshtriplett`` and ``@AzureMarker.``

This is my first draft, so feedback is welcome.

A few issues that definitely need feedback:

1. Naming. ``@joshtriplett`` called this `from_bytes_with_internal_nul` on Zulip, but after staring at all of the available methods, I believe that this function is probably what end users want (rather than the existing fn `from_bytes_with_nul`). Giving it a simpler name (**`from_bytes`**) implies that this should be their first choice.
2. Should I add a similar method on `CString` that accepts `Vec<u8>`? I'd assume the answer is probably yes, but I figured I'd try to get early feedback before making this change bigger.
3. What should the error type look like? I made a unit struct since `CStr::from_bytes` can only fail in one obvious way, but if I need to do this for `CString` as well then that one may want to return `FromVecWithNulError`. And maybe that should dictate the shape of the `CStr` error type also?

Also, cc ``@poliorcetics`` who wrote #73139 containing similar fns.

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