Add Cursor::seek_end and audit the codebase to use seek_start more

zed/src/sum_tree/cursor.rs

@@ -49,6 +49,16 @@ where
         &self.seek_dimension
     }
 
+    pub fn seek_end(&self, cx: &<T::Summary as Summary>::Context) -> S {
+        if let Some(item_summary) = self.item_summary() {
+            let mut end = self.seek_start().clone();
+            end.add_summary(item_summary, cx);
+            end
+        } else {
+            self.seek_start().clone()
+        }
+    }
+
     pub fn start(&self) -> &U {
         &self.sum_dimension
     }