To turn any struct into a composite element, you can implement a render
method with the following signature:

```rs
fn render<V: View>(&mut self, view: &mut V, cx: &mut ViewContext<V>) -> AnyElement<V>;
```

Then add #[derive(Element)] to the struct definition.

This will make it easier to introduce higher-level components that are
expressed in terms of other elements. Instead of calling functions that
return elements, we can now make any struct into an element fairly
easily. The advantage is that we can use method chaining to express
optional state on these components, and they blend in better with other
elements.

cc @mikayla-maki @osiewicz @iamnbutler 

Release Notes:

- N/A