Add instances for Profunctor and Traversing.

[freckletonj]

This is my first pass at adding Profunctor and Traversing instances.

I'm not 100% positive they're correct, though, some hand-rolled experiments seem to match my intuitions so far.

My interest in these instances stems from https://elvishjerricco.github.io/2017/03/10/profunctors-arrows-and-static-analysis.html. In it he builds a tree from FreeTraversing and in breaking it back down is able to build batched effects from singular effects. I'm hoping these instances will help me explore that possibility, we'll see!

Related: #224

[freckletonj]

updated!

So I'm not sure I understand the tradeoff between strict and lazy state. I would guess that laziness might be necessary for ArrowLoop or MonadFix to work properly... But I'm not sure.

[turion]

The difference is whether in a state transition the internal state (i.e. the current MSF continuation) is evaluated to WHNF. In most cases this is done automatically on the next tick because it is needed then.

[freckletonj]

@turion It's a humble PR, but it should be g2g now!
Thanks for your help writing it (the StateT thing was brilliant), and I'm happy to accept further advice.

[turion]

Some small comments

[turion]

You should definitely squash the commits ;)

[freckletonj]

Good recommendations. And I squashed the commits, but if you haven't seen it yet, I tend to use github's Merge Squash feature that does that automatically once right at the end when you merge, it's kinda nice too.

Any other issues you'd like a hand with?

[turion]

Looking great! @ivanperez-keera Is this fine to merge for you? I'm only asking since this introduces one dependency (profunctors).

Good recommendations. And I squashed the commits, but if you haven't seen it yet, I tend to use github's Merge Squash feature that does that automatically once right at the end when you merge, it's kinda nice too.

Right, I've just never used it. And I don't know what the commit message will be in that case.

Any other issues you'd like a hand with?

Any that are open, really :)

[freckletonj]

It actually gives you a prompt to write a commit message.

Cool I'll take a look at open issues :)

[ivanperez-keera]

I'll take a look and get back to you.

[miguel-negrao]

I believe this would make mapMaybeS and mapMSF special cases of traverse'. Should also work for Vector, Seq, vector-sized's Vector, etc. I find it quite useful.