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

Make gpui pass clippy

crates/gpui/src/text_system/line.rs

@@ -25,6 +25,7 @@ pub struct ShapedLine {
 
 impl ShapedLine {
     /// The length of the line in utf-8 bytes.
+    #[allow(clippy::len_without_is_empty)]
     pub fn len(&self) -> usize {
         self.layout.len
     }
@@ -58,6 +59,7 @@ pub struct WrappedLine {
 }
 
 impl WrappedLine {
+    #[allow(clippy::len_without_is_empty)]
     pub fn len(&self) -> usize {
         self.layout.len()
     }

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
     }