function digit2permission (_d) {
  const d = parseInt10(_d)
  const arr = d.toString(2).split('')
  const len = arr.length
  for (let a = len - 3, i = 0; i < a; i++) {
    arr.unshift('0')
  }
  const [rr, ww, xx] = arr
  const read = rr === '1'
  const write = ww === '1'
  const exec = xx === '1'
  return {
    read,
    write,
    exec
  }
}
digit to permission
@param {string} _d
@return {object} {read: Boolean, write: Boolean, exec: Boolean}