mark runtime as UnwindSafe

An alternative would be to mark bit of state as runtime safe, but as
Runtime directly contains a RefCell we need to mark it as a whole
anyway!

src/runtime.rs

@@ -35,6 +35,8 @@ pub struct Runtime<DB: Database> {
     shared_state: Arc<SharedState<DB>>,
 }
 
+impl<DB> std::panic::RefUnwindSafe for Runtime<DB> where DB: Database {}
+
 impl<DB> Default for Runtime<DB>
 where
     DB: Database,

tests/panic_safely.rs

@@ -63,3 +63,9 @@ fn should_panic_safely() {
     let result = panic::catch_unwind(AssertUnwindSafe(|| db.panic_safely()));
     assert!(result.is_ok())
 }
+
+#[test]
+fn storages_are_unwind_safe() {
+    fn check_unwind_safe<T: std::panic::UnwindSafe>() {}
+    check_unwind_safe::<&DatabaseStruct>();
+}