Import only used class methods of Foldable.

On GHC 7.8, the Foldable class contains also null and length, which
conflicts with Data.Sequence{null,length}.
It is perfectly fine to import class methods...

...without specifying the class, see Haskell 2010 5.2.1.
That allows us to get rid of some conditional includes.

Nevetheless, we still conditionally include foldr', as we do not
use it for base <4.8.