Switch Arc<Mutex<Keymap>> to Rc<RefCell<Keymap>>, a relic of the GPUI2 port.

Make gpui pass clippy

crates/gpui/src/gpui.rs

@@ -26,6 +26,9 @@
 //! TODO!(docs): Wrap up with a conclusion and links to other places? Zed / GPUI website?
 //! Discord for chatting about it? Other tutorials or references?
 
+// #![deny(missing_docs)]
+#![allow(clippy::type_complexity)]
+
 #[macro_use]
 mod action;
 mod app;