Constants on keccak256 usage

The idea came from a discussion with @0xPhaze. We should convert to constant calls to `keccak256` that are constants
```solidity
contract A{

function f() public returns(uint){
return uint(keccak256("something"));
}
}
```

This will be useful for the IR/echidna/proxy patterns

The issue names no implementation files, tests, or entry points. Start by locating Slither's handling of Solidity keccak256 expressions and the IR paths relevant to constant evaluation; define the supported constant forms and verify the result against the IR, Echidna, and proxy patterns named in the issue.

Written by the indexing model from the issue text.