[Deliverable] Deploy RLN smart contract to a Layer 2 mainnet

[jm-clius]

Description

The RLN smart contract should be deployed to a Layer 2 mainnet.

The contract is specified as output of #205
The contract implementation is described in #228

A first step here may be to deploy to a Layer 2 testnet first. However, we could deploy a test contract to a Layer 2 mainnet directly if it's easier.

Note that a test contract does not have to be productionised/audited before deployment to mainnet. However, eventually a production version of the contract will be deployed, after the auditing/revision described in #229 has been completed.