Need methods for traversing nodes, including unignored nodes

[mwcampbell]

Belatedly opening an issue for this. #3 is addressing this; I just need to review it.

[DataTriny]

Some custom iterators that are part of #3 currently yield NodeId when they should return Node<'_>,. This force us to perform more hash table lookups due to the fact that the Iterator trait is not able to yield items that reference the custom iterator struct itself. It appears that this is a well-known issue of current Rust, and that Generic Associated Types could improve the situation.
Therefore, I would suggest keeping this issue open, or opening a dedicated one so we can track this optimization opportunity.

[mwcampbell]

See #39 for a discussion on the lifetime challenges.