Add the snark complexity that we need for semaphore in miximus

[barryWhiteHat]

It would be nice to add this extra complexity so that we have a single general snark that can be used for mixing plus a bunch of other anon signaling stuff.

This would include adding

1. external nullifiers
2. signal
3. signal_variables

We would also need another hash of external_nullifier and nullifier. Tho this hash function is already added https://github.com/HarryR/ethsnarks/issues/72#issuecomment-436457227