merkle proofs: Fix get_helper_indices

[wemeetagain]

The current implementation of get_helper_indices will include unneeded indices if both direct children are not included in the proof (either as leaves or helpers/sisters).

eg: get_helper_indices([8,9,10,11,12]) should return [13, 7], but returns [13, 7, 3]
3 is unnecessary b/c it can be computed from 12, 13, 7.

Instead of only looking at direct children, all helper indices in a leaf -> root path should be filtered out, since any indices in a path can always be computed with a valid proof.

[protolambda]

Thanks @wemeetagain, this is more readable than the previous version, and corrects the path nodes set difference 👍
And apologies for the late review, I was too busy with devcon 😅