This renames `std::io::IoVec` to `std::io::IoSlice` and `std::io::IoVecMut` to `std::io::IoSliceMut`, and stabilizes `std::io::IoSlice`, `std::io::IoSliceMut`, `std::io::Read::read_vectored`, and `std::io::Write::write_vectored`. Closes #58452
AsRawFd