Add actor model

[Patrick Smith]

See GenServer from Elixir

protocol Actor {
  associatedtype MessageIn
  associatedtype MessageOut

  func handle(message: MessageIn) -> Deferred<MessageOut>
}

struct ActorID : Hashable {
  var identifier: String
}

class Supervisor {
  func start<A: Actor>(actor: A, identifier: String) -> Deffered<ActorID>
  func find<A: Actor>(id: ActorID) -> Deferred<A>
  func stop(id: ActorID) -> Deferred<()>
  func message<A: ActorID>(id: ActorID, message: A.MessageIn) -> Deferred<A.MessageOut>
}


struct Cache : Actor {
  enum MessageIn {
    case setKey(key: String, value: Data, expires: Date?)
    case setKeyPath(keyPath: [String], value: String, expires: Date?)
    case getKey(key: String)
    case getKeyPath(keyPath: [String])
    case fetchKey(key: String)
  }

  enum MessageOut {
    case ok
    case value(value: Data?)
  }

  enum Error {
    case keyPathNotFound(keyPath: [String])
  }

  private var values: [String: Data]

  func handle(message: MessageIn) -> Deferred<MessageOut> {
    …
  }
}