Remove all orphan instances

[treeowl]

• Remove all orphan instances.

• One of the unions definitions used foldl; make it use foldl'
  instead.

Closes #52.

[treeowl]

@konsumlamm This PR moves a lot of things around, so I'm pretty much blocked on everything else till it goes in.

[konsumlamm]

Are the Functor, Fodlable and Traversable instances of Down used anywhere? Also, I think a better module name instead of Data.PQueue.Prio.Max.Down would be Data.PQueue.Down, since it's not specific to MaxQueue or MaxPQueue.

[treeowl]

Are the Functor, Fodlable and Traversable instances of Down used anywhere?

I don't know.

Also, I think a better module name instead of Data.PQueue.Prio.Max.Down would be Data.PQueue.Down, since it's not specific to MaxQueue or MaxPQueue.

Good point.

[treeowl]

Traversable Down wasn't used, so I removed it. Functor Down and Foldable Down are currently used. We probably don't actually want to use Foldable Down (see #55), but we'll deal with that in a different PR. The Functor instance is used by things like mapKeys and mapKeysMonotonic. These are pretty inefficient no matter what we do (short of making all the trees lazy, which I'd very much rather not do), and I'm not optimistic that we can improve them much by reducing dictionary costs, but I guess we could try. I renamed the Down module Data.PQueue.Internal.Down, since it really is an internal thing.