[MLton] share bug

[Stephen Weeks]

> Sadly,  with  the  patch in it still died in the same way: the first GC after
> the first share with a failed assertion IsInFromSpace.
> 
> I added the GC_display calls before I ran it.  What should I look for?

Please send them.  I wanted to confirm the presence of the
intergenerational pointer.

It would also be worth adding a print statement to make sure that the
card mark in maybeSharePointer actually ran.

To speed up testing, perhaps it would be worth running with -debug
false, but explicitly add after the share a call to the GC and then to
the GC invariant (you will need a new import for this).