Enabling slashing spammers in RLN-Relay

[staheri14]

Problem

So far, the rln-relay membership group by implementation had an insertion-only structure. Member deletion was not supported mostly since the killic lib lacked the corresponding API. But this issue is resolved in the zero-kit, hence now is the time to implement the slashing part as well.

This issue is to capture the necessary steps:

• Updating the membership contract to accept slashed keys and to emit deletion events (RFC update + nim update)
• Identifying the secret id key of the spammers and sending a tx to the contract to remove them
• Listening to the deletion events emitted from the contract and updating the tree
• Dogfooding