Eq and Ord instances for Prio queues

[treeowl]

• Make the Eq and Ord instances for Prio queues compare the queues in fully sorted form—that is, as key-value maps. This seems to be the only way to make these instances make any real kind of sense.

• Document the "nondeterministic" nature of Prio queues.

[treeowl]

@konsumlamm Any comments?

[treeowl]

I really want the Eq instance to actually mean something. I think the original idea, when it was (mistakenly) thought that insertBehind could be implemented efficiently, was that Eq would be based on the order elements come out of the queue, and merge would be nondeterministic. But since insertBehind was a failure, I think we really want to make Eq equality of queues-as-maps, with merges considered deterministic. It's fine to require Ord. Is the instance really useful? Maybe not very, but the current one is nonsense. Let's do this please.

[treeowl]

Yes this should close #78.