countable-1.0: Countable, Searchable, Finite, Empty classes

• class Countable, for countable types
• class AtLeastOneCountable, for countable types that have at least one value
• class InfiniteCountable, for infinite countable types
• class Searchable, for types that can be searched over. This turns out to include some infinite types, see http://math.andrej.com/2007/09/28/seemingly-impossible-functional-programs/.
• class Finite, for finite types
• class Empty, for empty types
• data Nothing, an empty type

Some orphan instances:

• (Searchable a,Eq b) => Eq (a -> b)
• (Finite t) => Foldable ((->) t)
• (Finite a) => Traversable ((->) a)
• (Show a,Finite a,Show b) => Show (a -> b)