ThingBuf breaks a basic invariant under self-requeue test

[nodakai]

    #[test]
    fn test_() {
            fn to_vec<T: Clone + Default, const N: usize>(q: &StaticThingBuf<T, N>) -> Vec<T> {  // inaccurate
                let mut ret = vec![];
                while let Some(v) = q.pop() {
                    ret.push(v);
                }
                ret
            }
            const N: usize = 4;

            let q0 = StaticThingBuf::<u8, N>::new();
            for i in 0..N {
                q0.push(i as u8).expect("new enqueue");
            }
            eprintln!("start!");

            std::thread::scope(|sc| {
                for i in 0..2 {
                    sc.spawn(|| {
                        for k in 0..1000_000 {
                            if let Some(v) = q0.pop() {
                                q0.push(v).unwrap_or_else(|v| panic!("{}: q0 -> q0: {}, {:?}", k, v.into_inner(), to_vec(&q0)));
                            }
                        }
                    });
                }
            });

Confirmed an implementation based on a mutex, and another based on ring-channel didn't panic