sum_tree: Replace rayon with futures (#41586)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Co-authored by: Kate <kate@zed.dev>

crates/gpui/src/executor.rs

@@ -342,7 +342,7 @@ impl BackgroundExecutor {
     /// for all of them to complete before returning.
     pub async fn scoped<'scope, F>(&self, scheduler: F)
     where
-        F: FnOnce(&mut Scope<'scope>),
+        F: for<'a> FnOnce(&'a mut Scope<'scope>),
     {
         let mut scope = Scope::new(self.clone());
         (scheduler)(&mut scope);