Require Applicative (g s) as a superclass of FieldGrammar

[Bodigrim]

I think we can finally act on the old TODO, cutting a bit of boilerplate.

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[leana8959]

Do we need quantified constraints? I just removed these because it doesn't work well with type families in my exact print prototype.

[Bodigrim]

We would not be able to constrain FieldGrammar by forall s. Applicative (g s) without {-# LANGUAGE QuantifiedConstraints #-} enabled. Why does it mess up your type families?

(This PR is just a quality-of-life thing, I'm prepared to abandon it if it gets in the way of more important projects)

[leana8959]

My bad, I just tried to apply your changes here to exact print prototype and it worked fine. Last time I had trouble with QuantifiedConstraints was because the quantified constraint was a type family, which GHC can't deal with. In this case s is not a type family.

It's all good for me :)