Fix display of panic message in recursive panic.

library/core/src/panic.rs

@@ -157,4 +157,9 @@ pub unsafe trait PanicPayload: crate::fmt::Display {
 
     /// Just borrow the contents.
     fn get(&mut self) -> &(dyn Any + Send);
+
+    /// Try to borrow the contents as `&str`, if possible without doing any allocations.
+    fn as_str(&mut self) -> Option<&str> {
+        None
+    }
 }