Consider making genValidStructurally the default

[NorfairKing]

Currently the default implementation for GenValid is

genValid = genUnchecked `suchThat` isValid

Pros:

1. It's always correct, even if isValid explicitly doesn't require validity of a subpart.
   For example in

newtype InvalidRational = InvalidRational Rational

instance Validity InvalidRational where
  validate = trivialValidation

Admittedly, this is a pretty rare occurrence.

2. It does not require a Generic instance (as long as GenUnchecked didn't need one either).

The alternative is to have the default implementation be:

genValid = genValidStructurally

Pros: It's closer to what you want, usually. Eventhough an override is still necessary when you want genValidStructurallyWithoutExtraChecking.
Cons: It's not always correct, see the first pro of the current approach.

[NorfairKing]

It turns out you don't actually want this.