Provide `GenericQueuePair` and `GenericCompletionQueue` for dynamic dispatch

[dragonJACson]

If users want to implement a function returns either a BasicQueuePair or an ExtendedQueuePair, they cannot use Box<dyn QueuePair> in return position, as the function start_post_send in trait QueuePair would return BasicPostSendGuard and ExtendedPostSendGuard for the corresponding QueuePair type, which makes it impossible to consturct a Box<dyn QueuePair> type.

Thus, we need to provide a GenericQueuePair to users, it may look like this

pub enum GenericQueuePair {
    Basic(BasicQueuePair),
    Extended(ExtendedQueuePair),
}

impl QueuePair for GenericQueuePair {
    todo!();
}

impl From<BasicQueuePair> for GenericQueuePair {
    todo!();
}

impl From<ExtendedQueuePair> for GenericQueuePair {
    todo!();
}

Which should align the usage for both ExtendedQueuePair and BasicQueuePair

[dragonJACson]

Close with #41