Implement Default for AssertUnwindSafe

Trait impls are still insta-stable yeah...?

library/core/src/panic/unwind_safe.rs

@@ -279,6 +279,13 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     }
 }
 
+#[stable(feature = "assertunwindsafe_default", since = "1.62.0")]
+impl<T: Default> Default for AssertUnwindSafe<T> {
+    fn default() -> Self {
+        Self(Default::default())
+    }
+}
+
 #[stable(feature = "futures_api", since = "1.36.0")]
 impl<F: Future> Future for AssertUnwindSafe<F> {
     type Output = F::Output;