Stabilize the core::panic module

`std::panic` is already stable.

`core::panic::PanicInfo` and `core::panic::Location` are stable
and can be used through that path because of a bug
in stability checking: rust-lang#15702

src/libcore/panic.rs

@@ -1,8 +1,6 @@
 //! Panic support in the standard library.
 
-#![unstable(feature = "core_panic_info",
-            reason = "newly available in libcore",
-            issue = "44489")]
+#![stable(feature = "core_panic_info", since = "1.41.0")]
 
 use crate::any::Any;
 use crate::fmt;