* Generate redacted event enums and implement corresponding event structs
* Enable the *EventContent derives to generate redacted events
Most redacted event code is now generated by the *EventContent derive
macro. The exception are any content structs with the custom_redaction
attribute. This leaves implementing up to the user.
* Add redact method to Redaction/CustomEventContent
* Add accessor methods for redacted event enums
* Add RedactedEventContent trait and super traits to match EventContent
