Make child_iter in Flows not borrow the whole flow

[pcwalton]

It would be nice if child_iter didn't borrow the whole flow. Rust is being conservative here, because it doesn't know that the structure of the flow tree is guaranteed to be immutable during layout and is trying to guard against iterator invalidation. However, we already have to guarantee that the structure of the flow tree is immutable for parallel layout to be safe, so the iterator invalidation guarantees on child_iter aren't buying us anything.