Can we rework Function to get more instances?

Currently,

class Function a where
  function :: (a -> b) -> (a :-> b)

Unfortunately, I don't see any way to write anything like

instance ... => Function (Fun a b)

Maybe I'm wrong, but that's how it appears. This is sad, because we can't generate arbitrary higher-order functions. I'm wondering if it's possible to rearrange things to make this possible. The first idea is that perhaps we should remove the codomain from consideration and really focus hard on the domain. What does a type have to look like to be a Function instance? Well, basically, it has to look like one of these:

1. A type whose values can be listed
2. An algebraic combination of Function instances
3. A type that is isomorphic to an existing Function instance

I suspect it may be possible to express :-> with such a representation, but I'm not sure.

No files or tests are named. Start with the Function class and the Fun a b and :-e representations referenced in the issue, then inspect how function generation currently handles domains and codomains. Done means a settled design and supporting coverage for higher-order function generation.