CSE: generate code & storage for addresses in contract field

[palinatolmach]

As suggested by @PetarMax, the next step for CSE requires ensuring that code and storage are in place for addresses/accounts corresponding to contract fields.

It is to do with calls to addresses that don’t exist in the accounts. If we know that a storage slot of contract X we’re analysing holds an address of another account addrY coming from another contract Y, then we also know that account with address addrY should be in the storage of X and we also know the structure of the storage of Y.

For me, this should amount to:

• creating a fresh symbolic address addrY and putting it into the appropriate storage slot of X;
• calling the function that understands storage layout recursively on contract Y and putting whatever it returns in the account with address addrY.

It would be better to test this feature with non-test functions. There are some examples already in the codebase, specifically in this file.

An additional challenge comes from the fact that a field can be an interface but not a contract, in which case we can't run CSE since the we don't know what the addres we're calling has been instantiated with, so perhaps we can just leave it as a known limitation.