Untested manual tree walking version

I tested this slightly.  It runs through files.  But I haven't really tested it for bugs, etc.  It seems about 2x faster than the other one I was working with.  But entirely possible it is skipping things.

I did this because `Walk` is supposedly inefficient for large trees.  So I wrote some dedicated code just for trimming.

It probably has bugs.  But it is worth looking at just to see how you can manually walk the tree and keep things relatively simple.