Auto merge of #98324 - conradludgate:write-vectored-vec, r=Mark-Simulacrum

Auto merge of #98324 - conradludgate:write-vectored-vec, r=Mark-Simulacrum

attempt to optimise vectored write

benchmarked:

old:
```
test io::cursor::tests::bench_write_vec                     ... bench:          68 ns/iter (+/- 2)
test io::cursor::tests::bench_write_vec_vectored            ... bench:         913 ns/iter (+/- 31)
```

new:
```
test io::cursor::tests::bench_write_vec                     ... bench:          64 ns/iter (+/- 0)
test io::cursor::tests::bench_write_vec_vectored            ... bench:         747 ns/iter (+/- 27)
```

More unsafe than I wanted (and less gains) in the end, but it still does the job