Add arbitrum specific gateway contracts

[shalzz]

This is the native bridge implementation.

Some unsolved problems:

• Transferring ETH for paying the arbitrationCost after bridging:
  1.) This is inherently more complex than just state bridging (involves a bunch of error handling and refund handling which is very bridge specific and may not be refund claimable from non-EOA accounts)
  2.) This is especially true for the xdai bridge since eth<->xdai transfer is not intuitive

• Calculation of the arbitration cost in relation to the numOfJurors:
  1.) The same logic for this from the KlerosCore would need to be re-implemented here.
  2.) This would most likely require a governance function that updates the jurorFees per court on the gateways whenever there is a governance proposal to so on the KlerosCore contract as well.
  For now, the arbitration cost is just a constant i.e it does not scale with the numOfJurors.

[jaybuidl]

Shaleen there is no need for such complex inheritance, in particular diamond inheritance is generally an anti-pattern which may yield unexpected results due to the C3 linearization (a call to a super can be substituted by a call to a sibling).

Favor composition over inheritance: BaseXGateway does not need to inherit from ILXBridge, from an OO point of view, a gateway has a bridge seems more correct than a gateway is a bridge. So ILXBridge can just be a contract variable passed during initialization of BaseXGateway.

[shalzz]

Well, this is not diamond inheritance

[jaybuidl]

        IL1Bridge
        /       \
ArbL1Bridge   BaseForeignGateway
        \       /
     EthereumGateway

Am I missing anything?

[shalzz]

        IL1Bridge
        /       \
ArbL1Bridge   BaseForeignGateway
        \       /
     EthereumGateway

Am I missing anything?

IL1Bridge is an interface. So I wouldn't really count that as an inherited contract.
BaseForeignGateway is an abstract contract that need a IL1Bridge implementation. ArbL1Bridge provides that implementation.

[jaybuidl]

Right IL1Bridge is an interface. I still think that it can simplified bearing in mind that a Gateway is not a Bridge, it breaks the abstraction of the bridge which the gateway provides. To me BaseForeignGateway has a IL1Bridge. In this case the gateway would still need to expose the getSubmissionPrice() function but it can be arranged.

That would give:

 IL1Bridge ⬥-------- BaseForeignGateway
     ▲                       ▲
     |                       |
ArbL1Bridge           EthereumGateway

[shalzz]

The implementation would definitely change while solving the open problems I mentioned above which are bigger issues IMO.

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[jaybuidl]

Not feature complete but good enough to integrate and test the existing features.