New Data.Foldable methods

See http://thread.gmane.org/gmane.comp.lang.haskell.libraries/22828

The current version of the proposal at time of writing:

Add the following to Foldable:

• length — often stored as a separate field for //O(1)// access

• null — called very often and slightly faster than foldr (\_ _ -> False) True for some containers

• toList — may be the identity or nearly so, and this fact may not be statically available to the foldr/id rule

• sum, product — may be cached internally in tree nodes for fast update, giving //O(1)// access; may be calculated in parallel.

• maximum, minimum — //O(n)// for a search tree; //O(1)// for a finger tree.

• elem — //O(log n)// for search trees; likely better for hash tables and such.

• *Don't** add anything to Traversable, because scanl1 and scanr1 are not worth the extra dictionary weight.