Concurrent persistent stack

Fast, concurrent and easy to use implementation of persistent stack.

Operations cost

• push - O(1)
• pop - O(1)
• clone - O(1)
• iter next - O(1)

Example

```rust use persistent_stack::PersistentStack;

let mut s1 = PersistentStack::new(); s1.push(1);

let mut s2 = s1.clone(); // O(1) operation

std::thread::spawn(move || { s2.push(2); // PersistentStack values can be shared safely assert_eq!(s2.iter().copied().collect::>(), vec![2, 1]);

std::thread::sleep(std::time::Duration::from_millis(20));

s2.push(4);
assert_eq!(s2.iter().copied().collect::<Vec<_>>(), vec![4, 2, 1]);

assert_eq!(s2.pop(), Ok(4)); // We can also pop values from stack

});

s1.push(3); assert_eq!(s1.iter().copied().collect::>(), vec![3, 1]);

std::thread::sleep(std::time::Duration::from_millis(20));

s1.push(5); assert_eq!(s1.iter().copied().collect::>(), vec![5, 3, 1]); ```