async function walk (dir, visitor) {
  const filenames = await fs.readdirAsync(dir);
  await* filenames.map(async _filename => {
    const filename = path.join(dir, _filename);
    const stat = await fs.statAsync(filename);
    if (stat && stat.isDirectory()) {
      await walk(filename, visitor);
    } else {
      visitor(filename, stat);
    }
  });
}