Optimism L2 - bridge

[krebernisak]

• Add L1 & L2 contracts required for the Optimism token bridge (solc v0.7)
  • Technical details on the token bridge can be found on Notion | Optimism OVM (Optimistic Rollup) - Token Bridge
• Make L1/L2 Gateways an ERC677Receiver so transferAndCall can be used to avoid approving tokens before deposit/withdraw
• Add Optimism integration test
  • L1 -> L2 -> L1 deposit-wihdraw integration test
  • L2 withdrawals safety check integration test (blocking contract withdrawals but allowing EOA contracts to withdraw)
• Expose methods like deployGateways to deploy the L1<->L2 bridge that consumers might find useful.
• Add proxy deployment for the bridge contracts (L1 & L2 gateways)
  • Add @openzeppelin/contracts-upgradeable, the upgradeable variant of OpenZeppelin Contracts, meant for use in upgradeable contracts.

TODO:

• Define genesis ProxyEOA.sol codehash in OVM_EOACodeHashSet.sol
• Document the reasoning behind "unsafe" bridge operations
• Document upgradebility and bridge extension scenarios
  • Add deployment scenario (LinkTokenChild first, bridge second) integration test
• Get feedback
• Polish & finalize contracts
• Audit & approve

[krebernisak]

Closing this in favor of using the new Optimism standard bridge v2.