There are several TODOs here, but the code works for the two test cases
I created. Both of these avoid making extension nodes, which are going
away, and this code may need some tweaks to support values inside branch
nodes.

Per review comments

Per review comments

Per review comments

Per review comments

IteratorState is any of:
 - Start at the beginning of the trie
 - Start at a specific key value of the trie
 - Continue after a saved node and parents

See the inline comments for more details.

TODO:
 - Implement StartAtBegininng (and tests)
 - If a key is provided that doesn't exist, start iterating at the next
   one (if we really need this)
 - If an error is returned, we should next return None, then start from
   the beginning. This makes the resumption after an error a little
   cleaner. We'd need another IteratorState for this.
 - Remove extension nodes (depends on other diffs)

This was an artifact from an earlier change. Reverted back to original.

Not sure how this got in there. Reverting it.

Seems like we can just reference self and it works!

Removed unnecessary clone
Added additional comments
Added TODO for a few things:
 - possible to return a reference to the value instead of a copy
 - should handle case of branch nodes with values while traversing

Adding a randomized key-value test after extension nodes are removed is
also recommended.

This walks down the leftmost branch every time, recording the path down,
then figuring out what the key and value is at the bottom, then resumes.

Also refactors get_key_from_parents, as this is now done 3 times in the
code, sometimes when we add a leaf key value, and other times when not.

Per review comments

From review comments

Moved the comments about the behavior at the end of the stream to doc
comments on the iterator.

Per review comments.

Per review comments

Not 100% sure this is what the reviewer asked for, but this is okay too.

Per review comments

Per review comments

You have to add one only if it wasn't None

Added cases of starting in the middle and at the last key

Per review comments

The loop to calculate the next found position and address can be a
little simpler using enumerate, skip, and filter_map.

Also removed incorrect comment and comment about DiskAddress::is_null