Introduces ScreenViewFactoryFinder

[rjrjr]

Fixes #594, which was about how impractical it is to wrap the ViewRegistry
interface, by introducing a higher level interface that's easy to wrap.

The problem is that the fundamental method is getFactoryFor(KClass), but
lots of crucial behavior (e.g. AndroidViewRendering /
AndroidScreenRendering) is in the getFactoryForRendering extension
method. But if we change the fundamental method to be instance based instead
of type based, we bring back a lot of potential complexity to ViewRegistry
that the original type-based choice very intentionally restricted.

To have our cake and eat it too, we move the extension method to a new
ViewEnvironment service interface, ScreenViewFactoryFinder. Ta da,
totally customizable, with the defaults totally built in.

[steve-the-edwards]

Looks good but confused on the kdoc description on one point.

[steve-the-edwards]

I don't know if I understand this last sentence about [EnvironmentScreen] being the example of a customization of the [ViewEnvironment] entry. You mean using an [EnvironmentScreen] to extend the [ViewEnvironment] by adding a new map? This seems slightly different than 'customizing like any other [ViewEnvironment] entry'. E.g. I can change the map values wherever I have access to the viewEnvironment, using [EnvironmentScreen] seems like something else. Am I misunderstanding?

[rjrjr]

I'm just saying that EnvironmentScreen at the root of an app is the easiest way to do it. It's confusing, I'll just delete that phrase.

[rjrjr]

/**
 * [ViewEnvironment] service object used by [Screen.buildView] to find the right
 * [ScreenViewFactory]. The default implementation makes [AndroidScreen] work
 * and provides default bindings for [NamedScreen], [EnvironmentScreen], [BackStackScreen],
 * etc.
 *
 * Here is how this hook could be used to provide a custom view to handle [BackStackScreen]:
 *
 *    object MyViewFactory : ScreenViewFactory<BackStackScreen<*>>
 *    by ManualScreenViewFactory(
 *      type = BackStackScreen::class,
 *      viewConstructor = { initialRendering, initialEnv, context, _ ->
 *        MyBackStackContainer(context)
 *          .apply {
 *            layoutParams = (LayoutParams(MATCH_PARENT, MATCH_PARENT))
 *            bindShowRendering(initialRendering, initialEnv, ::update)
 *          }
 *      }
 *    )
 *
 *    object MyFinder : ScreenViewFactoryFinder {
 *      @Suppress("UNCHECKED_CAST")
 *      if (rendering is BackStackScreen<*>)
 *        return MyViewFactory as ScreenViewFactory<ScreenT>
 *      return super.getViewFactoryForRendering(environment, rendering)
 *    }
 *
 *    class MyViewModel(savedState: SavedStateHandle) : ViewModel() {
 *      val renderings: StateFlow<MyRootRendering> by lazy {
 *        val customized = ViewEnvironment() + (ScreenViewFactoryFinder to MyFinder)
 *        renderWorkflowIn(
 *          workflow = MyRootWorkflow.withEnvironment(customized),
 *          scope = viewModelScope,
 *          savedStateHandle = savedState
 *        )
 *      }
 *    }
 */

[rjrjr]

To verify during Gradle-induced JCenter outage, running ./gradlew clean build connectedCheck locally