Implemented Cancellation Logic

[Gathin23]

Fix: #142

Completed the implementation in the contract but unable to complete the testcases

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[glihm]

Thank you @Gathin23 for this first pass.

As I mentioned, you should call the StarknetCore contract, and not re-implement what it already does.

So when someone is starting a cancelling, you just have to forward it to the StarknetCore contract. Same thing for the cancel. Now, we do want to ensure that a request can be cancelled ONLY if the request was not already processed on L2.

So as I mentioned, for that we need an indexer that provide such information to the contract on ethereum.

So for now, when you implement the function that is forwarding to StarknetCore contract the cancelling, you can add a simple check in a state variable that check if the request hash is cancellable (that always return false for now). And in an other PR we will figure out how we will have this info available. :+1:

I think the most reasonable way to do so without paying too much fees, is to have once a day for instance, all the requests that has been processed on L1, we send a Tx on L1 with all the hashes to ensure they are not cancellable. Or the opposite approach, by default nothing is cancellable, and the indexer only register the hashes of the request that failed on L2. This one seems better. :)