### Motivation:
Initially I wanted to make `SubscriberAttribute.asDictionary` work using `Codable`. However, this isn't the right refactor because we need the actual raw types to be encoded inside `UserDefaults`.
At least this refactor provides several improvements:
### Changes:
- `SubscriberAttribute` now has value semantics
- ⚠️ `SubscriberAttribute.isSynced` used to be mutable, with reference semantics, which was dangerous
- `SubscriberAttribute: Equatable` implementation is now automatic
- Cleaned up `SubscriberAttribute.asDictionary` implementation, extracting keys into an `enum`
- Moved `Dictionary` -> `SubscriberAttribute` creation to the type itself
- ⚠️ `SubscriberAttribute.init(dictionary:)` returns an `Optional` instead of using force-unwrap, avoiding potential crashes
- Added more serialization / deserialization tests
- Removed `SubscriberAttributesMarshaller` and moved method into `SubscriberAttribute` extension
- Moved `SubscriberAttributeDict` into `SubscriberAttribute.Dictionary`
NachoSoto