This is neither a type nor a value. Simplifies `addStrLit` as well as
the many places that switch on `InternPool.Key`.
This is a partial revert of bec29b9e498e08202679aa29a45dab2a06a69a1e.
bb526426e7Andrew Kelley committed on 6/11/2023, 3:47:58 AMparent870e384