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Eliminates the boring identity functor I

Generically lift the applicative functor f out of a structure.

Example:

Given a datatype parametrised over the functor Maybe, return Just the value if all Maybe fields are Just, thereby changing the functor to I. Nothing otherwise.

>>> :{
>>> data Person f = Person { name :: f String, age :: f Int }
>>> deriving (Show, Generic, GHC.Generic)
>>> :}
>>> gvalidate $ Person (Just "LeBoeuf") (Just 42)
Just (Person (I "LeBoeuf") (I 42))
>>> gvalidate $ Person Nothing (Just 69)
Nothing

Generically change the functor of a structure from I to f, given a constructor of f.

Morally the inverse of gvalidate, although the specialisation to the identity functor makes this trivial.

If 'a f' is a Semigroup, this can be used to apply defaults to a partial value.

>>> :{
>>> instance Semigroup (Person Last) where
>>> a <> b = Person { name = on (<>) name a b, age = on (<>) age a b }
>>> -- For convenience, also define 'Monoid'
>>> instance Monoid (Person Last) were
>>> mempty  = Person mempty mempty
>>> mappend = (<>)
>>> :}
>>> defaultPerson :: Person I
>>> defaultPerson = Person "Sandy" 28
>>> gvalidate $ ginvalidate pure defaultPerson <> Person (pure "Maguire") mempty
Just (Person (I "Maguire") (I 28)