Add FunctorWithIndex constraint to Representable?

```haskell
defaultimap ::
Representable f => (Rep f -> a -> b) -> f a -> f b
defaultimap f fa = tabulate (f <*> index fa)
```

proves that we can (modulo package issues) use

```haskell
class (FunctorWithIndex (Rep f) f, Distributive f) => Representable f where ...
```

The only potential downside (aside from package dep and compatibility issues) is that someone could theoretically use something other than `Rep f` as the index. This seems a bit unlikely, but not entirely impossible.

As for the packaging, I think it's *weird* that `XxxWithIndex` live in `lens`. Are the lensy defaults really worth tying these simple classes up with the behemoth?