Move "async move" a few characters to the left in cx.spawn() (#26758)

This is the core change:
https://github.com/zed-industries/zed/pull/26758/files#diff-044302c0d57147af17e68a0009fee3e8dcdfb4f32c27a915e70cfa80e987f765R1052

TODO:
- [x] Use AsyncFn instead of Fn() -> Future in GPUI spawn methods
- [x] Implement it in the whole app
- [x] Implement it in the debugger 
- [x] Glance at the RPC crate, and see if those box future methods can
be switched over. Answer: It can't directly, as you can't make an
AsyncFn* into a trait object. There's ways around that, but they're all
more complex than just keeping the code as is.
- [ ] Fix platform specific code

Release Notes:

- N/A

crates/assistant_tools/src/read_file_tool.rs

@@ -70,14 +70,14 @@ impl Tool for ReadFileTool {
             return Task::ready(Err(anyhow!("Path not found in project")));
         };
 
-        cx.spawn(|mut cx| async move {
+        cx.spawn(async move |cx| {
             let buffer = cx
                 .update(|cx| {
                     project.update(cx, |project, cx| project.open_buffer(project_path, cx))
                 })?
                 .await?;
 
-            let result = buffer.read_with(&cx, |buffer, _cx| {
+            let result = buffer.read_with(cx, |buffer, _cx| {
                 if buffer
                     .file()
                     .map_or(false, |file| file.disk_state().exists())
@@ -102,7 +102,7 @@ impl Tool for ReadFileTool {
                 }
             })??;
 
-            action_log.update(&mut cx, |log, cx| {
+            action_log.update(cx, |log, cx| {
                 log.buffer_read(buffer, cx);
             })?;