[MLton] MLton.GC.collect hangs when using MLton.Finalizable

[Florian Weimer]

* Wesley W. Terpstra:

> At any rate, this only mattered b/c my Ring implementation is not
> thread-safe. Even if the common sub-expression didn't happen, there
> would still be a problem: If I am walking the ring and a GC occurs, I
> could get trapped on a removed link. Wrapping the add/remove/fold
> methods in a MLton.Thread.atomically makes the problem go away.

By the way, does MLton run finanlizers in a separate thread?  I
probably should, otherwise the above approach could easily lead to
deadlocks.