Deprecate Foldable for Either

Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`. With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed. I think this instance is harmful and should be deprecated (and later removed) from base. There are similarly pointless Foldable instances as well. See a discussion one year ago, which was heated, but had no consequences. https://mail.haskell.org/pipermail/libraries/2016-February/026678.html -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel at gu.se http://www.cse.chalmers.se/~abela/