Return opaque iterator for nearest neighbor searches

[kylebarron]

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It would be nice to finish this but also ensure this isn't a performance regression. It feels like the iterator has to repeat some steps that the existing method doesn't.

Perhaps it would be better to introduce a state machine, so that the iterator can check between an enum of states like "search child nodes, search queue, etc".

But overall I want to get the main library released, so this will wait for the next version.