[MLton] space leak

[Matthew Fluet]

> > Has anyone else noticed a major slowdown in self-compiles?  I'm seeing
> > upwards on 99% GC time for passes after defunctorize.  I've watched
> > things with -verbose 3, and the IL sizes don't seem to be off.
>
> The self compiles that I ran last night with clean sources completed
> in roughly 670s, which is normal these days.  There haven't been any
> commits of note today, so it looks likely the problem is on your end.
> Maybe you can try with clean sources to see what's up.

O.k.  I'll keep investigating.