Make Validation contravariant

[dhoepelman]

A Validation<Animal> should be usable as a Validation<Cat>, since animalValidation.validate(cat) is valid.

Allowed:

val animalValidation = Validation<Animal> {}
// Can use for subtype
val catValidation: Validation<Cat> = animalValidation
val catValidation2 = Validation<Cat> { run(animalValidation) }

Compile error since it makes no sense (not every animal is a cat)

val animalValidation: Validation<Animal> = catValidation
val animalValidation = Validation<Animal> { run(catValidation) }