Add method to help with strict containers

I propose an `NFData` method (by some name)

```haskell
whnfIsNf :: proxy a -> Bool
whnfIsNf _ = False
```

Then we can have, for example,

```haskell
instance NFData Integer where
whnfIsNf _ = True
instance NFData a => NFData (Set a) where
whnfIsNf _ = whnfIsNf (Proxy :: Proxy a)
rnf
| whnfIsNf (Proxy :: Proxy a)
= \ !_ -> ()
| otherwise
= ....
```

This can avoid a bunch of unnecessary traversals.

Start by reviewing the NFData interface and the proposed Set instance, then examine how strict containers are currently traversed. The issue names no files or tests; done would require settling the method name and semantics, adding the relevant NFData support, and demonstrating that unnecessary traversals are avoided.

Written by the indexing model from the issue text.