Extract `wander`/`traverse'` as separate function

[mniip]

The library has a Traversing typeclass with two methods:

traverse' :: forall a b g. Traversable f => p a b -> p (f a) (f b)
wander :: forall s t a b. Traversal s t a b -> p a b -> p s t

These two methods have default implementations in terms of each other doing some rather non-trivial manipulations with Bazaar-the-Applicative and Bazaar-the-Traversable.

However these manipulations are rather useful in other contexts, for concrete a, b, and p which need not be Traversing itself, e.g. defining these two functions in terms of each other:

bulk :: forall f. Traversable f => P (f A) (f B)
bulkOf :: forall s t. Traversal s t A B -> P s t

One way is straightforward:

bulk = bulkOf traverse

To define bulkOf in terms of bulk we need an "inverse" of ($ traverse):

generalizeTraversal :: Profunctor p => p (Baz t b a) (Baz t b b) -> Traversal s t a b -> p s t
generalizeTraversal lift traversal = dimap (\s -> Baz $ flip traversal s) sold lift

Then we have:

bulkOf = generalizeTraversal bulk

wander t pab = generalizeTraversal (traverse' pab) t

generalizeTraversal . ($ traverse) = id
($ traverse) . generalizeTraversal = id

Some things up to discussion:

• This mechanism is ultimately useful in context of lenses, so maybe it should be in the lens package instead; however this package already makes use of the mechanism; however it has to reimplement its own versions of bazaars for this? maybe bazaars have to come here altogether?
• generalizeTraversal could take the monomorphic p (Baz t b a) (Baz t b b) or the quantified (forall f. Traversable f => p (f a) (f b)) to hide the nature of its operation.
• generalizeTraversal may not be a good name

[mniip]

After some thought I realized the function can actually be extracted from existing profunctors code like so:

generalizeTraversal
  :: Profunctor p
  => (forall f. Traversable f => p (f a) (f b)) -> Traversal s t a b -> p s t
generalizeTraversal p trav = dimap Identity runIdentity
  $ runCofreeTraversing $ wander trav $ CofreeTraversing p