indexM in Representable?

There should be a function called something like `indexM` in the Representable typeclass, with the type

``` haskell
indexM :: (Representable f, Monad m) => f a -> Rep f -> m a
```

Its default definition would simply be

``` haskell
indexM f k = return $ index f k
```

But for some `Representable`s, it could produce a more defined result, for the same reasons as in `Data.Vector`. An example for `Product`:

``` haskell
indexM (Pair a _) (Left i) = indexM a i
indexM (Pair _ b) (Right j) = indexM b j
```

The result passed to `return` contains no reference to the original `Product` value. This could be a win for people using `Representable`s for memoization.

No file or test is named. Locate the Representable typeclass, its index implementation, and the Product representation first; then compare the proposed behavior with Data.Vector. Done means the indexM API and its default and Product-specific behavior have been agreed and covered.

Written by the indexing model from the issue text.