Passing side-effects to rendering layer.

## What
Currently, there isn't a well-defined mechanism for passing side-effects to the rendering layer.

### Using `SingleLiveEvent` concept
```kotlin
class SideEffect(val value: Type) {
private var performed: Boolean = false
inline fun performIfNeeded(crossinline action: () -> Unit) {
if (!performed) {
performed = true
action()
}
}
}

renderModel.scrollToTopSideEffect?.performIfNeeded {
recyclerView.scrollToTop()
}
```

The limitation is that there cannot be multiple things trying to handle the same side-effect. The side-effect should always be handled by a single `RenderModel` consumer.

### Using `Relay` concept

Side effect relay type passed through `RenderModel`
```kotlin
data class MyFeatureRenderModel(
val scrollSideEffects: SideEffect
)
```

Rendering layer
```kotlin
class MyFeatureRenderView(): RenderView {
private val recyclerView: RecyclerView = ...
private val scrollEffectHandler = SideEffect.Handler { effect: ScrollPosition ->
recyclerView.scrollTo(effect.position)
}

override fun render = Renderer { model: MyFeatureRenderModel ->
scrollEffectHandler.attach(model.scrollSideEffects)
}
}
```

Potential issues could be a missed side-effect event due to the subscription lifecycle. An effect could happen before the rendering layer is subscribed and can receive it.

Start by locating the RenderModel and rendering-layer entry points, then compare the SingleLiveEvent and Relay concepts described in the issue. Define the subscription lifecycle and single-consumer behavior, including what should happen when an effect occurs before subscription; the issue provides no named files or tests.

Written by the indexing model from the issue text.