Add derive(Debug) for Producer and Consumer types.

[liamkinne]

Another library I'm using returns these types as an Err(T) and I can't use unwrap because they don't implement Debug.

[ithinuel]

What does it print ?
Would it be useful for it to print the queued' items along with the queue's address ?

[liamkinne]

@ithinuel output looks like:

Producer { bbq: 0x16f6664e0, pd: PhantomData<&()> }
Consumer { bbq: 0x16d9524e0, pd: PhantomData<&()> }

So that gives the address. At this point, I don't think printing the queue's contents would be a good idea since the Producer/Consumer doesn't actually own the data, just a pointer to the BBBuffer.

[liamkinne]

bump @jamesmunns

[jamesmunns]

I don't have any concerns, I'll let @ithinuel decide whether to merge + release.