function fromString(hexString) {
  var arr = ethersUtils.splitSignature(hexString);
  return {
    // convert "v" to hex
    v: "0x".concat(arr.v.toString(16)),
    r: arr.r,
    s: arr.s
  };
}
split signature-hex into parts
@param  {string} hexString
@return {{v: string, r: string, s: string}}