When deserializing Events, Event.DeserializationStrategy assumed that the d field was the last field to be observed. If it wasn't the last field, the deserialization could fail in two ways:

• the t and s fields were ignored if they came after the d field
• an exception was thrown if the op field came after the d field

To fix these possible failure cases, the deserialization logic has been changed to work in two steps:

1. decode all fields regardless of order, treating the d field as a plain JsonElement
2. after all fields have been decoded, construct an Event from the JsonElement, depending on the values of the op, t and s fields

The supertype of Event.DeserializationStrategy has also been changed from DeserializationStrategy<Event?> to DeserializationStrategy<Event> - deserialize no longer returns null in some cases of illegal event payloads, but throws exceptions instead.

Fixes #922