add push_mut and pop_mut for queues

[jafar75]

#1179
This PR attempts to provide a simplified version of push and pop for crossbeam-queue data structures. By having an exclusive reference, there is no need to do CAS operations and some other checks.

• compare_exchange operations are removed
• all atomic load and store operations are done with Relaxed ordering.
• No need for backoff

[jafar75]

About drop block inside pop_mut, I think it is possible to do as below:

if offset + 1 == BLOCK_CAP {
    for i in 0..BLOCK_CAP - 1 {
        let slot = (*block).slots.get_unchecked(i);
        slot.state.fetch_or(DESTROY, Ordering::Relaxed);
    }
    drop(Box::from_raw(block));
} else {
    slot.state.fetch_or(READ, Ordering::Relaxed);
}

But I don't sure it is better than using Block::destroy() or not.