Add `partition_into` as a function on Iterator

[steffahn]

@Nokel81

a.into_iter().split_into(|&x| x > 0, |x| t.push_back(x), |x| h.insert(x));

I’m wondering: it this so much better than

a.into_iter().for_each(|x| if *x > 0 {t.push_back(x)} else {h.insert(x)});

?

(Edit: IMHO it’s worse.)