• Add ToOne<> class to wrap related entities. See examples for details.
  (thanks @Buggaboo for jump-starting this).
• Add ToMany<> class to wrap related entities. See examples for details.
• Significantly improve Box read and write performance.
• Change Box.put() and putMany() - now also update given object's ID property.
  Note: If you previously put() the same new object instance multiple times without setting id, the object will now
  be inserted only the first time, and overwritten on subsequent puts of the same instance, its ID is not zero anymore.
• Change Box.putMany() and Query.FindIds() to return fixed-size lists.
• Change Box.GetMany() to return a fixed-size list by default, with an option to return a growable list.
• Change @Id() annotation to optional - recognized automatically if there's an int id field (case insensitive).
• Make observable.dart part of objectbox.dart exports, no need to import it separately.
• Expose PutMode - allowing semantics choice between put, update and insert.
• Hide internal classes not intended for general use (e.g. all Model* classes).
• Rename versionLib() to nativeLibraryVersion().
• Change TxMode enum values to lowercase.
• Remove flags from the Property() annotation.