Add rules for Data.Set.alterF

We want

```haskell
alterF (const m) k s
-- or even
alterF (`seq` m) k s
===>
seq s $
bool (delete k s) (insert_preserving k s) <$> m
```

to avoid worrying about whether the key was in the map when it doesn't matter.

We might also want to generalize the `Const` rule to accommodate other `Const`-like functors:

```haskell
data Var = X | Y
-- When
-- fmap (`seq` X) (f False) = fmap (`seq` Y) (f False)
-- and
-- fmap (`seq` X) (f True) = fmap (`seq` Y) (f True)
alterF f k s ===> (`seq` empty) <$> f (member k s)
```

I haven't worked out the exact strictness for this bit.