Use panic() instead of panic!() in some places in core.

library/core/src/lib.rs

@@ -141,6 +141,7 @@
 #![feature(const_type_id)]
 #![feature(const_type_name)]
 #![feature(const_default_impls)]
+#![feature(core_panic)]
 #![feature(duration_consts_float)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(ptr_metadata)]