Return opaque iterator for nearest neighbor searches

[kylebarron]

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.

[kylebarron]

for ref https://docs.rs/kdtree/latest/kdtree/kdtree/struct.NearestIter.html