Add explicit for-item mapping to nearest-neighbor set

[jonahharris]

Currently, write_indicators() iterates the vector of nearest neighbors using the index as the for_item, which is incorrect. While the ordering of operations in indicators() should ensure items_to_rescore is ordered by index accordance with item_id as generated (in FIFO order from the interactions), that appears not to be the case any longer. Instead of relying on underlying collections to be order-preserving, this instead returns a tuple with the for_item id along with the FnvHashSet of nearest neighbors from rescore().