Add zero-compromise directory iteration using getdents64

[SUPERCILEX]

Still need to add docs and tidy some stuff up. Plus #1854 will fix the extra formatting changes.

Benchmarks:

fn main() {
    let fd = open(
        env::args().nth(1).unwrap().as_str(),
        OFlag::O_RDONLY,
        Mode::empty(),
    )
    .unwrap();
    let fd = unsafe { OwnedFd::from_raw_fd(fd) };
    let mut buf = Vec::with_capacity(8192);

    // Do the same thing for Dir and read_dir
    let mut something: usize = 0;
    for file in RawDir::with_vec(fd.as_fd(), &mut buf) {
        something = something.wrapping_add(file.unwrap().name.to_bytes().len());
    }
    println!("{}", something);
}

$ ftzz g -n 1M -d 0 /tmp/ftzz-test
$ hyperfine "./dents /tmp/ftzz-test" "./dir /tmp/ftzz-test" "./stdlib /tmp/ftzz-test"                                                                               11/06/2022 04:43:44 PM
Benchmark 1: ./dents /tmp/ftzz-test
  Time (mean ± σ):     166.5 ms ±   1.4 ms    [User: 5.2 ms, System: 161.1 ms]
  Range (min … max):   164.2 ms … 169.4 ms    17 runs
 
Benchmark 2: ./dir /tmp/ftzz-test
  Time (mean ± σ):     202.7 ms ±   2.6 ms    [User: 42.2 ms, System: 159.9 ms]
  Range (min … max):   200.0 ms … 206.8 ms    14 runs
 
Benchmark 3: ./stdlib /tmp/ftzz-test
  Time (mean ± σ):     210.0 ms ±   3.9 ms    [User: 51.7 ms, System: 158.0 ms]
  Range (min … max):   204.0 ms … 220.0 ms    14 runs
 
Summary
  './dents /tmp/ftzz-test' ran
    1.22 ± 0.02 times faster than './dir /tmp/ftzz-test'
    1.26 ± 0.03 times faster than './stdlib /tmp/ftzz-test'

[SUPERCILEX]

@asomers This should be ready to rock and roll.

[SUPERCILEX]

What's the policy on breaking changes like this? Option<Type> -> Type

I could undo this by matching on Unknown and returning None.

[SUPERCILEX]

Closing this as it's wrong anyway. See my rustix implementation for the latest and greatest: https://github.com/bytecodealliance/rustix/blob/main/src/fs/raw_dir.rs