Deprecate Foldable for Either

[Henning Thielemann]

On Thu, 2 Mar 2017, Andreas Abel wrote:

> 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]`.

That is, he did (concat (xs :: [Either a [b]]))? Does GHC actually accept 
that? If at all, it would not have to do with Foldable.