Put slicing syntax behind a feature gate.

[breaking-change]

If you are using slicing syntax you will need to add #![feature(slicing_syntax)] to your crate.

src/libcoretest/lib.rs

@@ -7,7 +7,7 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
-#![feature(globs, unsafe_destructor, macro_rules)]
+#![feature(globs, unsafe_destructor, macro_rules, slicing_syntax)]
 
 extern crate core;
 extern crate test;