Punches up composite rendering API and demonstrates nav logging.

[rjrjr]

Introduces a few interfaces to make it easier to navigate composite rendering structures, useful for both production logging and workflow unit testing.

Updates some samples (hellobackbutton, ravenapp and poetryapp) to demonstrate their use for navigation logging.

For example, from this naive snippet in HelloBackButtonActivity:

    }.onEach {
      Timber.i("Navigated to %s", it.unwrap())
    }

we get this very useful logcat:

Navigated to HelloBackButtonScreen(message=Able, onClick=Function0<kotlin.Unit>, onBackPressed=null)
Navigated to HelloBackButtonScreen(message=Baker, onClick=Function0<kotlin.Unit>, onBackPressed=Function0<kotlin.Unit>)
Navigated to HelloBackButtonScreen(message=Charlie, onClick=Function0<kotlin.Unit>, onBackPressed=Function0<kotlin.Unit>)
Navigated to HelloBackButtonScreen(message=Baker, onClick=Function0<kotlin.Unit>, onBackPressed=Function0<kotlin.Unit>)
Navigated to HelloBackButtonScreen(message=Able, onClick=Function0<kotlin.Unit>, onBackPressed=null)
Navigated to AlertOverlay(buttons={POSITIVE=I'm Positive, NEGATIVE=Negatory}, message=Are you sure you want to do this thing?, title=, cancelable=true, onEvent=Function1<com.squareup.workflow1.ui.navigation.AlertOverlay$Event, kotlin.Unit>)

Emphasis on "naive". To keep the samples simple I didn't debounce anything, so in a more complex app you'd actually see a "Navigate to…" each time you update something in place. But that's a trivial fix:

    ).onEach {
      NavLogger.log(it)
    }
  }

  private object NavLogger {
    var lastKey = ""

    fun log(dest: Any) {
      val unwrapped = dest.unwrap()
      Compatible.keyFor(unwrapped).takeIf { it != lastKey }?.let { newKey ->
        Timber.i("Navigated to: %s", unwrapped)
        lastKey = newKey
      }
    }
  }