Add the reach function to the Strong class

[guaraqe]

This adds the reach function to the Strong class. It has the following type:

reach :: (forall f. Functor f => (a -> f b) -> (s -> f t)) -> p a b -> p s t

It has two uses:

• It allows the definition of more efficient Profunctor-transformation functions, since it can avoid the construction of tuples demanded by the other functions in this class.

• It acts as a compatibility layer with lenses, without the need for extra dependencies. The first parameter of the function is essentially the definition of a Lens. The default definition of 'first'', for example, is:

  first' = reach _1

It is mutually defined in terms of second', and first' was modified to be defined in terms of reach, which means that no existing code should break.

I had to add a Functor dependency to the Kleisli instance for Strong to satify older versions of GHC. I don't know if this is the best solution.

[guaraqe]

I just checked and it seems that the same error due to Functor and Monad happens in the Traversing class, that was not touched in this PR. If compatibility with GHC 7.8 is not required, I can remove the workaround I mentioned above in the Kleisli instance.