async function walk (dir, visitor) {
  const filenames = await fs.readdirAsync(dir)

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