[plt-scheme] macro stepper enhancements

>
>> 2. Going a step back takes a long time, I guess because the entire 
>> computation from beginning to end is re-computed? Can all the steps 
>> be cached along the way instead?
>
> They are. The steps are only recomputed when you change the hiding 
> policy. The time is being taken up by pretty-printing, coloring, 
> clickback-attaching, etc. I'll check to see if any of the latter parts 
> are happening outside of edit-sequences, but other than that I don't 
> know of any way to speed it up.
>

If you can't speed it up can you provide '<- step 5' ? Or something like 
that.