Proposal: Add Applicative (and Monoid) instances in GHC.Generics

[Ryan Scott]

In response to each of your proposals:

> 1. Add an instance (Monoid c => Applicative (K1 i c))

This seems like a no-brainer to me. +1

> 2. Add helpers
>
>          gmempty :: (Generic a, Applicative (Rep a)) => a
>          gmempty = to (pure ())
>
>          gmappend :: (Generic a, Applicative (Rep a)) => a -> a -> a
>          gmappend a b = to (from a <*> from b)
>
>          -- also gpure, gap for generic Applicative

I'm weakly -1 on this. I don't think GHC.Generics should be a place
where we collect combinators for generically implementing various type
class methods. I'd prefer that these be left to downstream libraries,
especially since they're usually extremely straightforward to
implement and wouldn't cross the Fairbairn threshold for me.

> 3. Add Monoid instances

I think I support this idea. But just to be sure we're on the same
page: can you say which instances in particular you're adding, and how
you'd implement them?

Ryan S.