Commits on Aug 21, 2019

1. Adds Kotlin Sink, makeActionSink, makeEventSink. …

   This PR introduces:
   
    * a Kotlin `Sink` class matching Swift's
    * `RenderContext.makeActionSink` matching Swift's `makeSink(ofActionType)`
    * `RenderContext.makeEventSink` matching Swift's `makeSink(ofEventType)`
   
   and deprecates:
   
    * `WorkflowAction.enterState`
    * `WorkflowAction.emitOutput`
    * `WorkflowAction.modifyState`
    * `RenderContext.onEvent`
    * `EventHandler`
   
   Kotlin apps are now able to make custom WorkflowAction class hierarchies in
   the style common in Swift apps. This is made possible by the new
   `WorkflowAction.apply` method, analagous to the same method in Swift.
   Where the Swift version has an `inout StateT` function argument,
   Kotlin's is called against a `Mutator<StateT>` receiver.
   
   This allows us to implement action types like the following, where the null
   returns indicate that no output is to be emitted.
   
   ```
     sealed class Action : WorkflowAction<Movement, Nothing> {
       class StartMoving(private val direction: Direction) : Action() {
         override fun Mutator<Movement>.apply(): Nothing? {
           state += direction
           return null
         }
       }
   
       class StopMoving(private val direction: Direction) : Action() {
         override fun Mutator<Movement>.apply(): Nothing? {
           state -= direction
           return null
         }
       }
     }
   ```
   
   The convention is to declare these `Action` types inside workflows, instead of
   `Event` type trees in rendering classes. And instead of creating `EventHandler`
   instances to be used by renderings, we create `Sink` instances:
   
   ```
     override fun render(
       input: TakeTurnsInput,
       state: Turn,
       context: RenderContext<Turn, CompletedGame>
     ): GamePlayScreen {
       val sink = context.makeActionSink<Action>()
   
       return GamePlayScreen(
           playerInfo = input.playerInfo,
           gameState = state,
           onQuit = { sink.send(Quit) },
           onClick = { row, col -> sink.send(TakeSquare(row, col)) }
       )
     }
   ```

   

   rjrjr committed Aug 21, 2019
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