This PR fixes `ST.Ref.ptrEq` to act as described in the docs. This fixes
two bugs:
1. The recent `IO.RealWorld` elimination PR overlooked this function
(afaik this is the only one),
causing its return value to be generally wrong.
2. The implementation of `ptrEq` would previously always consider two
different cells with pointer
equivalent value to be pointer equal. However, the function is supposed
to check whether two
`Ref` are the same cell, not whether the contained elements are.
