message: Add roundtrip test with serde_json
  This will be necessary to ensure we can fix without breaking
  backwards compatibility.
diff: @@ -45,6 +45,15 @@ mod serde_tests {
  let _mat: CsMat<u8> = serde_json::from_str(&json_mat).unwrap();
  }
  + #[test]
  + fn roundtrip_matrix() {
  + let json_mat = r#"{ "storage": "CSR", "ncols": 10, "nrows": 2, "indptr": [0, 2, 3], "indices": [4, 6, 9], "data": [4, 1, 8] }"#;
  + let mat: CsMat<u8> = serde_json::from_str(&json_mat).unwrap();
  + let serialized = serde_json::to_string(&mat).unwrap();
  + let mat2 = serde_json::from_str(&serialized).unwrap();
  + assert_eq!(mat, mat2);
  + }
  +
  #[test]
  fn invalid_matrices() {
  // indices not sorted
  