Simplify component state

[jstarry]

Problem

The internal component state machine is over complicated. Empty state and Destroyed state are essentially the same. The Processing state is an implementation detail and therefore unnecessary.

Changes

• Simplify the component state machine
  • The component state is now option wrapped. None means the component doesn't exist (rather than using Empty or Destroyed).
  • We also eagerly create the component rather than using a delayed create task. I don't anticipate this causing any issues.
  • Lastly, Yew will no longer immediately call Component::render after creating a component. This revealed an assumption in a yew-functional test where we expected the component to receive an update event before being destroyed.
• Rework functional tests to not rely on the component rendering before being destroyed
• Add internal "force" component update mechanism

[jstarry]

@ZainlessBrombie If you have time to review how this affects functional components, that'd be great!

My understanding is that this change makes eager component deletion more consistent since we don't couple component creation with rendering anymore. And so, if a parent decides to render and then not-render a child component, it's possible that a functional component will not be able to update itself to apply an effect before being destroyed.

I think we could take this one step further by splitting component update tasks from component render tasks. Which would mean that child components would not be rendered at all if they were immediately deleted.