b0cbd4e5f3
Stabilize `seek_seek_relative`
This PR stabilizes `seek_seek_relative`:
```rust
// std::io::Seek
trait Seek {
fn seek_relative(&mut self, offset: i64) -> Result<()>;
}
```
<br>
Tracking issue: https://github.com/rust-lang/rust/issues/117374.
Implementation PR: https://github.com/rust-lang/rust/pull/116750.
FCPs already completed in the tracking issue.
Closes https://github.com/rust-lang/rust/issues/117374.
r? libs-api