Throw core::panic!("message") as &str instead of String.

This makes it consistent with std::panic!("message"), which also throws
a &str, not a String.

library/std/src/lib.rs

@@ -259,6 +259,7 @@
 #![feature(exhaustive_patterns)]
 #![feature(extend_one)]
 #![feature(external_doc)]
+#![feature(fmt_as_str)]
 #![feature(fn_traits)]
 #![feature(format_args_nl)]
 #![feature(gen_future)]