Flatten RedwoodView and RedwoodView.Root

[squarejesse]

This is a step in simplifying the complex interaction between these two classes. It also removes an unnecessary layer from the view tree.

We also had a potential bug on hot reloads, where the previous content was detached but not necessarily removed. With this change we clear the child views by default on a hot reload.

---

• CHANGELOG.md's "Unreleased" section has been updated, if applicable.

[squarejesse]

Let the base class just clear everything

[squarejesse]

For a follow-up, I’ll refactor these arguments to match what the content can actually be:

sealed class ContentState {
  /** 
   * This view has no content. This is the initial state of the view.
   * 
   * This may occur if the content is gracefully stopped, in which case
   * the view may continue to display the previous content, though it
   * will not be interactive.
   */
  object Detached : ContentState

  /** 
   * Content isn't ready yet. It’s busy downloading, launching, or computing 
   * the initial view tree.
   */
  object Loading : ContentState()

  /** 
   * Content is ready and responsive to interactions. 
   */
  data class Attached(val loadCount: Int): ContentState()

  /** 
   * This view has crashed due to a bug.
   */
  data class Crashed(uncaughtException: Throwable?): ContentState()
}