uuid method
Implementation
String uuid() {
final b = randomBytes(16);
b[6] = (b[6] & 0x0f) | 0x40;
b[8] = (b[8] & 0x3f) | 0x80;
final h = HEX.encode(b);
return '${h.substring(0, 8)}-${h.substring(8, 12)}-${h.substring(12, 16)}-${h.substring(16, 20)}-${h.substring(20)}';
}