Use fstatat() in DirEntry::metadata on Apple platforms

[jesseschalken]

Apple supports fstatat on macOS >=10.10 (source), and according to Platform Support the oldest supported version is 10.12.

Google says iOS >=10 supports fstatat but doesn't provide a source. *-apple-ios says the minimum supported iOS version is 10.0.

Unsure about tvOS, watchOS and visionOS, hoping CI can confirm this.

I am testing with fastdu which is effectively a stress test for DirEntry::metadata. In one test this provides a 1.13x speedup.

$ hyperfine --warmup 1 'target/release/fastdu testdir' 'fastdu testdir'
Benchmark 1: target/release/fastdu testdir
  Time (mean ± σ):     154.6 ms ±  17.4 ms    [User: 31.7 ms, System: 187.6 ms]
  Range (min … max):   148.4 ms … 225.5 ms    19 runs
 
Benchmark 2: fastdu testdir
  Time (mean ± σ):     175.3 ms ±  15.8 ms    [User: 50.0 ms, System: 196.2 ms]
  Range (min … max):   165.4 ms … 211.7 ms    17 runs
 
Summary
  target/release/fastdu testdir ran
    1.13 ± 0.16 times faster than fastdu testdir

You can also reproduce a speedup with a program like this (providing a directory with many entries):

fn main() {
    let args: Vec<_> = std::env::args_os().collect();
    let dir: PathBuf = args[1].clone().into();

    for entry in dir.read_dir().as_mut().unwrap() {
        let entry = entry.as_ref().unwrap();
        let metadata = entry.metadata();
        let metadata = metadata.as_ref().unwrap();
        println!("{} {}", metadata.len(), entry.file_name().display());
    }
}

$ hyperfine './target/release/main testdir' './main testdir'
Benchmark 1: ./target/release/main testdir
  Time (mean ± σ):     148.3 ms ±   5.2 ms    [User: 23.1 ms, System: 122.9 ms]
  Range (min … max):   145.2 ms … 167.2 ms    19 runs
 
Benchmark 2: ./main testdir
  Time (mean ± σ):     164.4 ms ±   9.5 ms    [User: 32.6 ms, System: 128.8 ms]
  Range (min … max):   158.5 ms … 199.5 ms    17 runs
 
Summary
  ./target/release/main testdir ran
    1.11 ± 0.07 times faster than ./main testdir

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