More effect combinators

[huntc]

Effect combinators added for emitting events and replying; both with the same abilities as and_then to inspect behaviour and state, and act in accordance with the previous result.

Here's a contrived example that I tried out:

emit_event(TempEvent::TemperatureUpdated { temp: 10 })
    .and_then_reply(|_: &Self, new_state, prev_result| {
        let result = if prev_result.is_ok() {
            if let (Some(new_state), Ok(_)) = (new_state, prev_result) {
                Ok(Some((reply_to, new_state.temp)))
            } else {
                Ok(None)
            }
        } else {
            prev_result.map(|_| None)
        };

        async { result }
    })
    .and_then_emit_event(|_, _, _| async {
        Ok(Some(TempEvent::TemperatureUpdated { temp: 20 }))
    })
    .boxed()

[patriknw]

LGTM

[patriknw]

Unrelated to this PR, but we need to test what happens when deleting an entity. I assume it is possible to create a new entity with with same entity_id and then it will start from seq_nr 1 again. That will probably be confusing to the jvm side when producing events. Such events will be seen as duplicates.

[huntc]

Hmm. Interesting. We might be ok... Certainly worth testing.