Proposal: Add an atomic IORef replacement operation

Currently, atomicWriteIORef is implemented using atomicModifyIORef. This is not ideal: the latter allocates memory while the former shouldn't need to. In the process of writing alternative implementations, I realized that we can efficiently support an atomic operation that replaces the value in an IORef, returning the previous value. I propose that we add, to Data.IORef, a function to do so: atomicReplaceIORef :: IORef a -> a -> IO a