static byte[] hexToBytes(S s) {
  int n = l(s) / 2;
  byte[] bytes = new byte[n];
  for (int i = 0; i < n; i++) {
    S hex = substring(s, i*2, i*2+2);
    try {
      bytes[i] = (byte) parseHexByte(hex);
    } catch {
      fail("Bad hex byte: " + quote(hex) + " at " + i*2 + "/" + l(s));
    }
  }
  ret bytes;
}

Began life as a copy of #1004113