limit checks

[Matthew Fluet]

> So as far as the limit check framework is concerned, I would like to
> break the problem into two pieces -- 
>   1. where to put limit check points points
>   2. given a placement, figure out the right amount at each point
> 
> In MLton, there are two things to fix with (1), the unnatural loop bug
> and we need to add limit checks at loop exits so that limit checks
> aren't hoisted into loops.  I hope that both of those are doable with
> loop forests.  With (2), we need to fix the dfs, or go with some other
> algorithm.

O.k.  By fixing the "unnatural loop bug" do you mean changing
contification so that it doesn't create unnatural loops or do you mean
changing limit-check.fun so that it inserts limit checks at loop headers
of unnatural loops?  The second should be possible with loop forests.