Allow some Record introspection

[cedvdb]

The idea is to have a way to find undefined state

Something like this:

typedef BunnyRecord = PartialRecord<Bunny>; // allows 0 fields in Record
class Bunny {
  final bool isCute;
  final String? name;
  
  Bunny copyWith( BunnyRecord fields) {
    return Bunny(
      isCute: fields.hasPropertyDefined(BunnyRecord.properties.isCute) ? fields.isCute : this.isCute,
      name: fields.hasPropertyDefined(BunnyRecord.properties.name)  ? fields.name : this.name,
    );
  }
}

Admitedly this #140 is be a better solution in the case of copy with but not in other cases like serialization

   final  a = jsonEncode(<BunnyRecord>(isCute: true)); // {'isCute': true }
   final b = jsonEncode(<BunnyRecord>(isCute: true, name: null)); // {'isCute': true, 'name' : null }

The difference between a and b here becomes apparent when you send the json serialization as a database entry update.