Add a derive macro for Element

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

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.

crates/gpui/src/gpui.rs

@@ -26,7 +26,7 @@ pub mod color;
 pub mod json;
 pub mod keymap_matcher;
 pub mod platform;
-pub use gpui_macros::test;
+pub use gpui_macros::{test, Element};
 pub use window::{Axis, SizeConstraint, Vector2FExt, WindowContext};
 
 pub use anyhow;