Zed2 entities (#3189)

Adds an `Entity` trait for abstracting over `View`s and `Model`s, and
implements it for the `subscribe()` and `observe()` APIs.

The last commit also includes a fun experiment I added, using the
`Result` type to return the owned model handles back to the caller in
the case of downcast failure, inspired by the `binary_search*` methods.

crates/gpui2/src/app.rs

@@ -730,7 +730,7 @@ impl Context for AppContext {
 
     /// Update the entity referenced by the given model. The function is passed a mutable reference to the
     /// entity along with a `ModelContext` for the entity.
-    fn update_entity<T: 'static, R>(
+    fn update_model<T: 'static, R>(
         &mut self,
         model: &Model<T>,
         update: impl FnOnce(&mut T, &mut Self::ModelContext<'_, T>) -> R,