tryPushNode

fn tryPushNode(arena: *ArenaAllocator, node: *Node) PushResult {
    assert(node != node.next);
    if (@cmpxchgStrong( // strong because retrying means discarding a fitting node -> expensive
        ?*Node,
        &arena.state.used_list,
        node.next,
        node,
        .release, // syncs with acquire in failure path or `loadFirstNode`
        .acquire, // syncs with release in success path
    )) |old_node| {
        return .{ .failure = old_node };
    } else {
        return .success;
    }
}