fix: FSM actors to reply in cats-actors style

[PetrosPapapa]

This PR ensures the FSM actor replies to asks in the cats-actors way, i.e. by returning the responses directly to the caller of the ask instead of sending them as messages.

I tried to reconcile the original Akka implementation with the cats-actors style. The main issue is that classic Akka permits multiple responses for a single state change. These are accumulated in the State$replies list and sent as messages one by one on transition. More importantly, if you do not want any responses in a state change, you just leave that list empty by default.

In cats-actors, we only return a response once. For this reason, I opted to make the response type of the actor be a monoid. This covers all cases of not responding (yields Monoid$.empty), a single response, or multiple responses combined into 1.

The simplest way to handle this is to have the FSM actor response type be a list, but the user has the option of defining a custom response and its own Monoid.

We somewhat retain Akka's ability to respond by sending a message. We introduce a StateReplyType that controls how an FSM response is provided:

• ReturnResponse: Return the value in order to fulfil an ask.
• SendMessage: Send the response as a message to the original sender.
• BothMessageAndResponse: Both send and return the response.

Notes:

• The State$.replying method defaults to SendMessage.
• Added a State$.returning method that responds using ReturnResponse.
• Unlike Akka, we send a single message that combines all of the responses instead of a message for each response.

Breaking changes:

• The FSM actor is no longer an Actor but a ReplyingActor so you need to use actorSystem.replyingActorOf (unless you typecast it to make the response type Any).
• The response type of the FSM actor must be monoidal. Multiple responses are aggregated into one.
• A more conscious decision must be made when constructing a replying state by choosing to use State$.replying or State$.returning.

See updated unit tests for examples. The README is also updated.

requires suprnation/cats-actors#20 resolves suprnation/cats-actors#13

[PetrosPapapa]

Updated the PR and description to reflect the latest changes after discussing the way forward with @cloudmark .

[PetrosPapapa]

Will be merged in #22