RTree searches: return iterator for nearest neighbor search

[kylebarron]

Right now, our nearest neighbors implementation returns a materialized vec of all values that are within max_distance and max_results, but instead it would be cleaner to just return an iterator. I.e. this loop:

geo-index/src/rtree/trait.rs

Lines 195 to 206 in c5300f4

	// pop items from the queue
	while !queue.is_empty() && queue.peek().is_some_and(|val| (val.0.id & 1) != 0) {
	let dist = queue.peek().unwrap().0.dist;
	if dist > max_dist_squared {
	break 'outer;
	}
	let item = queue.pop().unwrap();
	results.push(item.0.id >> 1);
	if max_results.is_some_and(|max_results| results.len() == max_results) {
	break 'outer;
	}
	}

that results.push(item.0.id >> 1); should yield instead of return.