Update cargo to 2024 edition (#842)

Some effects of this change:
- New formatting changes across many files. No functionality changes
should occur from that.
- Calls to `set_env` are considered unsafe, since this only happens in
tests we wrap them in `unsafe` blocks

codex-rs/tui/src/chatwidget.rs

@@ -1,7 +1,7 @@
 use std::path::PathBuf;
+use std::sync::Arc;
 use std::sync::mpsc::SendError;
 use std::sync::mpsc::Sender;
-use std::sync::Arc;
 
 use codex_core::codex_wrapper::init_codex;
 use codex_core::config::Config;
@@ -17,8 +17,8 @@ use ratatui::layout::Layout;
 use ratatui::layout::Rect;
 use ratatui::widgets::Widget;
 use ratatui::widgets::WidgetRef;
-use tokio::sync::mpsc::unbounded_channel;
 use tokio::sync::mpsc::UnboundedSender;
+use tokio::sync::mpsc::unbounded_channel;
 
 use crate::app_event::AppEvent;
 use crate::bottom_pane::BottomPane;