intoiter: Add release method to OwnedRepr

Author: bluss

src/data_repr.rs

@@ -92,6 +92,13 @@ impl<A> OwnedRepr<A> {
         self.len = new_len;
     }
 
+    /// Return the length (number of elements in total)
+    pub(crate) fn release_all_elements(&mut self) -> usize {
+        let ret = self.len;
+        self.len = 0;
+        ret
+    }
+
     /// Cast self into equivalent repr of other element type
     ///
     /// ## Safety