perf: minimize iterations over children in `IntersectionIterator`

[kylebarron]

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I was reading through the code and noticed that this was a double for loop, but children2 doesn't depend on child1 at all, so it doesn't need to be in the for loop. You do have to collect the items into a vector because you can't use the iterator more than once, but seems better than recomputing it every loop iteration?

[urschrei]

Lgtm!

[urschrei]

Bors r=adamreichold

[bors[bot]]

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