Add json support for class Card

[simphotonics]

This enhancement request is related to this question on stackoverflow.

• Add a method toJson() or a getter toJson:

/// Returns a map representation of this.
  Map<String, dynamic> get toJson => {
        'due': due.toIso8601String(),
        'stability': stability,
        'difficulty': difficulty,
        'elapsedDays': elapsedDays,
        'scheduledDays': scheduledDays,
        'reps': reps,
        'state': state.toString(),
        'lastReview': lastReview.toIso8601String()
      };

• Add a constructor Card.fromJson:

/// Converts a map to an object of type `Card`.
factory Card.fromJson(Map<String, dynamic> json) {
    // Validate input
    if (json
        case {
          'due': String due,
          'stability': double stability,
          'difficulty': double difficulty,
          'elapsedDays': int elapsedDays,
          'scheduledDays': int scheduledDays,
          'reps': int reps,
          'state': String state,
          'lastReview': String lastReview
        }) {
      return Card()
        ..due = DateTime.parse(due)
        ..stability = stability
        ..difficulty = difficulty
        ..elapsedDays = elapsedDays
        ..scheduledDays = scheduledDays
        ..reps = reps
        ..state = State.values
            .firstWhere((element) => element.toString() == state.toString())
        ..lastReview = DateTime.parse(lastReview);
    } else {
      throw JsonUnsupportedObjectError(json,
          cause: 'In CardHelper.fromJson: '
              'Validation failed!');
    }
 }

• Override the == operator and the method hashCode such that two Card objects are equal if all their fields are equal (see package equatable). That way the original object and the de-serialized object are equal.

[sonbui00]

@simphotonics I added new code to support json. Please try in version 1.1.1