User can change venmo id during registration

[kcharbo3]

The venmo_actor_id 'reveal' outputs an array where everything is 0s except for where the venmo id is:

https://github.com/zkp2p/zk-p2p/blob/242724a2a8736814c90f27a8131af79ee001ca20/circuits-circom/regexes/venmo_actor_id.circom#L204-L207

So the registration circuit will call ShiftAndPack on this output array, to shift the array to the point the venmo id number starts. However, the number of shifts is up to the user. The user can input any number as the shift count as the signal venmo_actor_id_idx:

https://github.com/zkp2p/zk-p2p/blob/242724a2a8736814c90f27a8131af79ee001ca20/circuits-circom/venmo_registration.circom#L60-L69

As far as I know there are no constraints on this signal, so it's a degree of freedom for the user. I believe they can input different shifts to get possible venmo ids. For example, if the output array from venmo_actor_id is [0, 0, 0, 1, 2, 3, 4, 0, 0] and the actual id is [1, 2, 3, 4], then I believe the user could enter the shift as '5' (when it should be 4) to get an id of [2, 3, 4, 0].

Let me know if my thinking is correct here. If so, I believe we need some additional constraint on the input shift.