Why is there no Foldable instance for Either?

[Gabriel Gonzalez]

I'm surprised that `Either` does not have a `Foldable` instance of the form:

     import Data.Foldable
     import Data.Monoid

     instance Foldable (Either e) where
         foldMap f (Left  _) = mempty
         foldMap f (Right r) = f r

         foldr _ z (Left  _) = z
         foldr f z (Right r) = f r z

In other words, the above instance would behave like the `Maybe` 
`Foldable` instance, treating `Left` in the same way as `Nothing`.

This came up in the context of an issue raised on the `errors` package:

https://github.com/Gabriel439/Haskell-Errors-Library/issues/16

I could provide an orphan instance for `Either` in the `errors` package, 
but I wanted to ask if it was possible to incorporate the instance 
directly into `Data.Foldable`.