darcs patch: Make toList a member of Foldable

[Ross Paterson]

On Tue, Apr 27, 2010 at 11:53:47PM +0200, Eelis van der Weegen wrote:
> Thanks a lot for mentioning the RULES pragma, which I did not know
> about. It turns out that using it to specialize Foldable's toList for
> things like NonEmptyList works like a charm.

If the existing rules don't already do this for [], it needs fixing.

The instance for NonEmpty should be

  instance Foldable NonEmpty where
    foldr f x (NonEmpty h t) = f h (foldr f x t)

which would then rely on the optimization of the [] instance.