QueueSender<T>/QueueReceiver<T>: Send/Sync impls need `T: Send`

Affected versions of this crate unconditionally implemented Send/Sync for QueueSender<T>,
allowing to send non-Send T to other threads by invoking (&QueueSender<T>).send().

This fails to prevent users from creating data races by sending types like Rc<T> or Arc<Cell<T>> to other threads, which can lead to memory corruption.

The flaw was corrected in commit 1e462c3 by imposing T: Send to both Send/Sync impls for QueueSender<T>/QueueReceiver<T>.