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

Make gpui pass clippy

crates/gpui/src/text_system/line_layout.rs

@@ -162,6 +162,7 @@ pub struct WrapBoundary {
 }
 
 impl WrappedLineLayout {
+    #[allow(clippy::len_without_is_empty)]
     pub fn len(&self) -> usize {
         self.unwrapped_layout.len
     }