5fb8a39266
Stabilize checked slice->str conversion functions
This PR stabilizes the following APIs as `const` functions in Rust 1.63:
```rust
// core::str
pub const fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error>;
impl Utf8Error {
pub const fn valid_up_to(&self) -> usize;
pub const fn error_len(&self) -> Option<usize>;
}
```
Note that the `from_utf8_mut` function is not stabilized as unique references (`&mut _`) are [unstable in const context].
FCP: https://github.com/rust-lang/rust/issues/91006#issuecomment-1134593095
[unstable in const context]: https://github.com/rust-lang/rust/issues/57349