add Foldable and Traversable instances for Either a and (,) a

Agreed in January 2011, but not implemented:

http://thread.gmane.org/gmane.comp.lang.haskell.libraries/15196
http://thread.gmane.org/gmane.comp.lang.haskell.libraries/17686
http://thread.gmane.org/gmane.comp.lang.haskell.libraries/19594