Revisit *Calls API

[smonicas]

This PR revisits the calls API by changing it to be a list of IR operations, in that way the API should be more flexible and can be used in more places. One exception is for high level calls which is a tuple of Contract - IR because the target contract has some logic to be applied so i preferred to keep this logic in a single place where the high level calls are computed in the node so places that use it don't need to worry about it. One open question is if internal calls should contain solidity calls, this was the old behavior and i did not change it, i'm not sure why given that solidity calls have their own property. Updated some detectors to use the new API which should improve the performance as they don't have to iterate nodes or IRs.

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