Rollup merge of #74021 - 1011X:master, r=dtolnay

Rollup merge of #74021 - 1011X:master, r=dtolnay

impl Index<RangeFrom> for CStr

This change implements (partial) slicing for `CStr`.

Since a `CStr` must end in a null byte, it's not possible to trim from the right and still have a valid `CStr`. But, it *is* possible to trim from the left. This lets us be a bit more flexible and treat them more like strings.

```rust
let string = CStr::from_bytes_with_nul(b"Hello World!\0");
let result = CStr::from_bytes_with_nul(b"World!\0");
assert_eq!(&string[6..], result);
```