[project @ 1999-07-27 11:12:05 by simonmar]

add unofficial/undocumented function:

    mkWeakIORef :: IORef a -> IO () -> IO (Weak (IORef a))

for weak pointers to IORefs.  This avoids some of the problems with
inlining disturbing the semantics of mkWeak by making the weak key
point to the primitive MutVar instead of the box.