Add pagination combinators

[flupe]

For static-site generation purposes, it's required to provide combinators such as the following:

chunks :: Task m Int -> Task m [a] -> Task m [[a]]

(We have the above already, although change info may be incorrect).

But something along those lines would be nice:

paginate :: Task m [a] -> Task m [(a, These a a)]

paginate
  :: Task m [a] 
  -> (Task m a -> Task m (i, b))
  -> (Task m b -> Task m (These i i) -> c)
  -> Task m [c]

Again, more thinking should go into this.