Investiage using proxies upgrades for gates

[hgedia]

With the updated API, we pass gate_argument into the verify function of the placeholder_verifier contract. However, we want to enforce these gate arguments/libraries to implement a signature for the gate evaluation function.

There are two issues we need to address with this task :

1. Libraries cannot inherit in solidity. - So signature cannot be enforced.
2. Design patterns to support upgradability.