Don't reuse excerpt ids in MultiBuffer

This prevents anchors from swapping their ordering, which was causing issues in FoldMap.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/text/src/locator.rs

@@ -49,6 +49,30 @@ impl Default for Locator {
     }
 }
 
+impl sum_tree::Item for Locator {
+    type Summary = Locator;
+
+    fn summary(&self) -> Self::Summary {
+        self.clone()
+    }
+}
+
+impl sum_tree::KeyedItem for Locator {
+    type Key = Locator;
+
+    fn key(&self) -> Self::Key {
+        self.clone()
+    }
+}
+
+impl sum_tree::Summary for Locator {
+    type Context = ();
+
+    fn add_summary(&mut self, summary: &Self, _: &()) {
+        self.assign(summary);
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;