Detector idea: flag storage slot usage if contract is upgradeable

[bsamuels453]

Describe the desired feature

The idea here is that using EIP-7201 should be the recommended best-practice way of managing contract storage of upgradeable contracts. We can push best practice by flagging a finding on upgradeable contracts that store variables in the storage slots at the beginning of the contract (ex: variables listed in slither --print variable-order)

Upgradeable contracts can be vaguely identified by the presence of an initialize() function. This would limit false positives to factory-deployed contracts, confidence medium.

We can get better results by checking for inheritance of OZ's UUPSUpgradeable or similar contracts, confidence high.

Severity medium imo, but open to thoughts

[bsamuels453]

solc is gonna add this eip as a first class feature which will simplify this detector a bit: ethereum/solidity#597 (comment)