perf: replace pool hasher with xxh3

[JohnTitor]

Description

Implement content hashing with xxh3 to improve performance.

With this change, the below benchmark shows positive results:

• Baseline: 4.640 ms
• After: 2.765 ms (40.4% less time)

let tmp = TempDir::new().expect("tempdir");
write_files(tmp.path(), 2_000, 4 * 1024);

let mut watcher = PollWatcher::new(
   |_event| {},
   Config::default()
      .with_compare_contents(true)
      .with_manual_polling(),
)
.expect("create watcher");
watcher
   .watch(tmp.path(), RecursiveMode::Recursive)
   .expect("watch");

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