Potential additional superclass of Traversing?

Something that's `Traversing` should perhaps be, in addition to being `Strong` and `Choice`, also something that represents being able to go over a fixed number of values. If there were some kind of numbered `Vec`, I could see it going something like this:

```haskell
class Profunctor p => TraversingFixed p where
traverseFixed :: KnownNat n => p a b -> p (Vec n a) (Vec n b)
```

The other way I could see it happening is by using the fact that a `Distributive` always has the same number of values in it, because it's isomorphic to `x -> a` for some `x`:

```haskell
class Profunctor p => TraversingFixed p where
traverseDist :: (Traversable f, Distributive f) => p a b -> p (f a) (f b)
```

I honestly have no idea how to convert the former to the latter, though, or what to do if the `f` to be traversed is infinite, because infinite traversals in Haskell *can* be meaningful. I'm also not entirely sure how a hypothetical `wander` analogue of `traverseFixed` would go, since the point of `Applicative` is that it can combine an arbitrary number of fixed effects.

Start by reading the existing Traversing, Strong, Choice, and Distributive definitions and their laws. Compare the proposed fixed-size Vec and Distributive approaches, including the infinite-traversal question, before deciding whether a superclass is coherent. Done requires an agreed design and a concrete specification for the new abstraction.

Written by the indexing model from the issue text.