Fix alloc/ToString error

Author: bluss

src/error.rs

@@ -495,6 +495,8 @@ fn test_sizes() {
 
 #[test]
 fn test_encode_decode_format() {
+    use alloc::string::ToString;
+
     assert_eq!(
         ShapeError::invalid_axis(1, 0).to_string(),
         "ShapeError/OutOfBounds: out of bounds indexing; expected: 1, but got: 0");