External code can't access Consume fields, and thus can't implement Consumer

[daboross]

I ran into this when copying over the implementation from #137 into a crate outside of rubble - it looks like Consume, the type used for conveying results to Consumer, has private fields only accessible to rubble. Unless I'm missing a way to get into Consume::consume and Consume::result, this prevents outside users from implementing queues built to their needs.

I think this could be fixed pretty easily by either making fields public (maybe with an adding private () field for forward compatibility), or by adding accessors. If it's alright, I'll submit a PR - I mildly favor accessors, but since rubble's still pre-1.0, and Consume is fairly feature-complete for what it is, it probably doesn't matter a ton?

[jonas-schievink]

Oof, yeah, feel free to add accessors.

[daboross]

Will do then. Filed #148!