sorted_map: add is_empty

src/librustc_data_structures/sorted_map.rs

@@ -143,6 +143,11 @@ impl<K: Ord, V> SortedMap<K, V> {
         self.data.len()
     }
 
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     #[inline]
     pub fn range<R>(&self, range: R) -> &[(K, V)]
         where R: RangeBounds<K>