ArrayQueue: is it possible to push and pop and the same time?

[gootorov]

Essentially I'm looking for something like this:

use crossbeam_queue::ArrayQueue;

// allow a maximum of two elements.
let q = ArrayQueue::new(2);

assert_eq!(q.push_displacing(10), None);
assert_eq!(q.push_displacing(20), None);
assert_eq!(q.push_displacing(30), Some(10));

(not sure on the naming)

Is it possible to implement something like this in theory? (I guess it would require writing to two memory locations atomically at the same time, which there is no way to so, not sure)
If, however, it is possible, I'd be happy to open a PR given some hints how to properly implement it!

[brunocodutra]

We could in fact change the RingQueue I propose in #789 to return the oldest element instead of dropping it in place. Seems like a great idea to me in fact, I'll look into that hopefully later today.