function substringBytes(value, start, end) {
  let buf = Buffer.from(value);
  return buf.subarray(start, end).toString();
}
Takes a substring of a string based on byte offsets.
@private
@method substringBytes
@param {string} value : The input string.
@param {number} start : The byte index of the substring start.
@param {number} end : The byte index of the substring end.
@return {string} : The substring.