large compile times for mlyacc

[Stephen Weeks]

> I can't say for certain whether or not the stuff introduced in 1.21 slowed
> stuff down (I would think not, because the List.foreach -> List.fold
> should be almost the same loop).  My guess is that there really was a
> slow-down before rev 1.20 was checked in.
...
> So, something else between the release and now messed it up.
> If you've got the time, I suggest really rolling back to rev. 1.20 for
> directed-graph, deal with whatever type errors might come up (I don't
> think you'll actually see anything) and see how that goes.  I may have
> just screwed up trying to roll back by hand (there were some changes I
> thought would help; or at least not hurt).

I rolled back and that solved the problem.  I checked in the rollback
for now.  Matthew, hopefully you can figure out what in 1.21 caused
the slowdown.  In the meantime, self-compile times are reasonable
again, and will even smaller once we get better limit check
coalescing.