Modifiers for test data.

These types do things such as restricting the kind of test data that can be generated. They can be pattern-matched on in properties as a stylistic alternative to using explicit quantification.

Examples:

 -- Functions cannot be shown (but see Test.QuickCheck.Function)
 prop_TakeDropWhile (Blind p) (xs :: [A]) =
   takeWhile p xs ++ dropWhile p xs == xs

 prop_TakeDrop (NonNegative n) (xs :: [A]) =
   take n xs ++ drop n xs == xs

 -- cycle does not work for empty lists
 prop_Cycle (NonNegative n) (NonEmpty (xs :: [A])) =
   take n (cycle xs) == take n (xs ++ cycle xs)

 -- Instead of forAll orderedList
 prop_Sort (Ordered (xs :: [OrdA])) =
   sort xs == xs