Investigate doing the iterators using iterator composition

[rklaehn]

Write the iterators as a recursive tree traversal that returns iterators.

Now that lots of stuff is wrapped in an arc, the stack usage should be minimal, so you should be able to traverse really deep trees with a small stack. Something like a few 1000 deep would seem to be a good limit.

Note: this needs precise understanding of the stack usage, including tests, to make sure we can traverse large trees.