bugfix regarding recent master->vm merge rewrites history

[Andy Wingo]

Hi,

There was a problem in the conversion of subrs to double-cells, in which
the additional cells were not being marked. This was quite a number of
commits ago, on master, and having such a large period in which that bug
was present is not good for bisection. So I've revived the VM branch.
It's the same as master, except commit
325226dad9ab6f0488500e7381a5d1c07dc9ae91 was retroactively introduced in
the history.

Anyway, all this to say that VM has been rewritten, and I'd like to copy
it over to master. This is fine, but whenever you pull and get a "forced
update" you always wonder why that was -- and this is why. If no one
objects, I'll forcibly reset master to vm later this evening.

Sorry for the confusion. Play around in gitk if it's not clear to you:

  gitk origin/master...origin/vm

All of the merges are from when I was trying to figure out which patch
introduced the bug.
  
Cheers,

Andy
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