Proposal: Add a few extra members to Foldable and Traversable classes

To go along with Herbert Valerio Riedel's moving functions from Foldable and Traversable to Prelude, I realized that `null` and `size` make sense for all Foldables; Reid W. Barton noted that these can have optimized implementations for many containers. He also noted that scanl1 and scanr1 make sense for general Traversables. I therefore propose: Add `null` and `size` to the Foldable class. Default implementations: null = foldr (\_ _ -> False) True size = foldl' (\acc _ -> acc+1) 0 Add `scanl1` and `scanr1` to the Traversable class. Default implementations are a little less simple. David