\U00ABCD12 and \uABCD escapes are deprecated

[akosthekiss]

During builds and tests, output is cluttered with warnings about deprected unicode escapes. E.g.:

/home/akiss/devel/rust/src/libserialize/json.rs:2765:53: 2765:62 warning: \U00ABCD12 and \uABCD escapes are deprecated
/home/akiss/devel/rust/src/libserialize/json.rs:2765         check_encoder_for_simple!('\U0010ffff', "\"\U0010ffff\"");
                                                                                                         ^~~~~~~~~
/home/akiss/devel/rust/src/libserialize/json.rs:2765:53: 2765:62 help: use \u{ABCD12} escapes instead
/home/akiss/devel/rust/src/libserialize/json.rs:2765         check_encoder_for_simple!('\U0010ffff', "\"\U0010ffff\"");

/home/akiss/devel/rust/src/libcoretest/char.rs:229:18: 229:23 warning: \U00ABCD12 and \uABCD escapes are deprecated
/home/akiss/devel/rust/src/libcoretest/char.rs:229     assert_eq!('\u0300'.width(true),Some(0));
                                                                    ^~~~~
/home/akiss/devel/rust/src/libcoretest/char.rs:229:18: 229:23 help: use \u{ABCD12} escapes instead
/home/akiss/devel/rust/src/libcoretest/char.rs:229     assert_eq!('\u0300'.width(true),Some(0));
                                                                    ^~~~~
/home/akiss/devel/rust/src/libcoretest/str.rs:121:46: 121:55 warning: \U00ABCD12 and \uABCD escapes are deprecated
/home/akiss/devel/rust/src/libcoretest/str.rs:121     assert_eq!(Utf16Encoder::new(vec!['é', '\U0001F4A9'].into_iter()).collect::<Vec<u16>>(),
                                                                                               ^~~~~~~~~
/home/akiss/devel/rust/src/libcoretest/str.rs:121:46: 121:55 help: use \u{ABCD12} escapes instead
/home/akiss/devel/rust/src/libcoretest/str.rs:121     assert_eq!(Utf16Encoder::new(vec!['é', '\U0001F4A9'].into_iter()).collect::<Vec<u16>>(),

It would be useful to clean this up.