Switch to indexed applicatives

[treeowl]

• The original qtraverse wasn't powerful enough to implement
  qfoldMapDefault. Using an indexed applicative makes it so.
• Add qfoldMapDefault.
• Adjust the type of traverse_ to match.
• Remove the surprising QFunctor superclass of QFoldable.

Closes #10

[treeowl]

@echatav , here's my first little pull request. I have one or two more generalities to put in before I start porting things over from type-aligned (with some performance tweaks).

[echatav]

This looks isomorphic to ApQ t c x y since (OpQ c) y x is isomorphic to c x y, so seems redundant

[treeowl]

Redundant how? The basic idea is that it's possible to implement, say, a queue that enqueues on the left and dequeues on the right by wrapping the more usual version in QReverse. How do you propose to do it?

[treeowl]

Okay, I guess it can always be done using IxBackwards directly. I just stole the idea from Data.Functor.Reverse. I guess it's at least a useful demonstration of IxBackwards?

[echatav]

I got the isomorphism wrong, it's not ApQ, it's OpQ (t (OpQ c)). It's interesting because you could use it in reversePath to change the signature from

reversePath :: (QFoldable c, CFree path) => c p x y -> path (OpQ p) y x

to

reversePath :: (QFoldable c, CFree path) => c p x y -> ReverseQ path p x y

I haven't given thought to how to port queues since I never needed them :-/