client code should also valide field inputs

[bkomuves]

This is arguably a bug in zerokit, but any case, not only the smart contract, but the client code should also validate field-like inputs; eg. in membership_pda_seed().

Really, one should never ever use bytes to store field elements, apart from dealing with external input / output (use a proper field element type instead, even if in the worst case it's just a newtype wrapper), and only have a single validating conversion function from bytes into the proper type (which gives an error for invalid input).