encode method
Implementation
@override
Uint8List encode(Object value) {
final exception = Exception(
'Invalid value for type "$this": $value (${value.runtimeType})');
if (value is! String) throw exception;
var s = value.toLowerCase().trim();
if (s.length != 20) throw exception;
return IntType.encodeFromBigInt(BigInt.parse(s, radix: 16));
}