Class DropGuard

crates/libsy/src/core/algorithm.rs:1383–1383

Sets a flag when dropped, so we can observe whether the algorithm task was cancelled/dropped.

        // Sets a flag when dropped, so we can observe whether the algorithm task was
        // cancelled/dropped.
        struct DropGuard(Arc<AtomicBool>);
        impl Drop for DropGuard {
            fn drop(&mut self) {
                self.0.store(true, Ordering::SeqCst);