#36: verify circuit improvement

[lolaaimar]

PR Summary:

• Add assertions in verify circuit to check:
  • contract is initialized
  • caller's commitment belongs in the contract (if the caller can't commit, they shouldn't be able to verify either)
• Adds tests for the new assertions
• Modifies e2e example:
  • update contract with new assertions
  • updates complete flow example with an unauthorized call to mint to show it throws