itree.c: Add new "stateless" iterator code and post-order traversal

This still uses the old iterator code, but runs the new code
alongside to make sure they behave identically.

* src/itree.c (struct itree_iterator): New field `node`.
(itree_iterator_create): Give it a sane default value.
(itree_iterator_busy_p, itree_iterator_start, itree_iterator_finish):
Move down to the "iterator" section of the file.
(itree_iter_next_in_subtree, itree_iterator_first_node)
(itree_iter_next): New functions.
(itree_iterator_start): Initialize the new `node` field.
(itree_iterator_next): Add post-order case.
Call the new "stateless" `itree_iter_next` function and check that it
agrees.

* src/itree.h (enum itree_order): New value for post-order traversals.

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