Event behavior for evoq.

Events represent facts that have happened. They are: - Past tense: account_opened, money_deposited - Immutable once stored - Produced by aggregates in response to commands - Domain artifacts: atom keys, Erlang terms, stay inside bounded context

Required Callbacks

- event_type() -> atom() - new(Params) -> Event - to_map(Event) -> map()

Optional Callbacks

- from_map(Map) -> {ok, Event} | {error, Reason}