function arraySize (x) {
  const s = []

  while (Array.isArray(x)) {
    s.push(x.length)
    x = x[0]
  }

  return s
}
Calculate the size of a multi dimensional array.
This function checks the size of the first entry, it does not validate
whether all dimensions match. (use function `validate` for that)
@param {Array} x
@return {number[]} size