Factor the contract digest logic down to the Contract class

[JuanCoRo]

In solc_to_k.py, Constructor and Method are both child classes of Contract. However, both have the contract_up_to_date function, whereas the Contract class doesn't.
This can cause some redundancy, such as the logic introduced in #735 to determine whether the contract's digest is outdated¹.

In this particular case, instead of

any(
        not method.contract_up_to_date(Path(method.contract_digest))
        for contract in self.contracts.values()
        for method in contract.methods
    )

we could do something like

any(
       not contract.contract_up_to_date(Path(contract_digest))
       for contract in self.contracts.values()
   )

Taking into account that the digest files look like this:

{
    "methods": {
        "test%CounterTest.test_Increment()": {
            "method": "4c32041fa5a7c64776f36dab5ae8cc61588efc3e3fb09b3c95ed0917587687d6",
            "contract": "29c961bb216d9a5f16961d6a224fd6cb1feeb18b36e29e9a09c2733b0590a655"
        },
        "test%CounterTest.setUp()": {
            "method": "2c1737186c0be7909e9db344314dabfa10f5f492345ad17e87375ed13c1bcaa6",
            "contract": "29c961bb216d9a5f16961d6a224fd6cb1feeb18b36e29e9a09c2733b0590a655"
        },
        "test%CounterTest.testFuzz_SetNumber(uint256)": {
            "method": "323d5de4c923be8a393316579d3833bd4f00d4d81a5a85b3030a54efd2222e87",
            "contract": "29c961bb216d9a5f16961d6a224fd6cb1feeb18b36e29e9a09c2733b0590a655"
        }
    },
    "kompilation": "089cb7a3c95b4643e44147c41139b379b7a69797b3a8cb3d6a9e4acc41fb3bef",
    "kontrol": "0.1.32",
    "foundry": "a7794d1db53643ecf7e89e8be01ca0fcad6ca2cf381e68f4fdbe70d1be6934f9"
}

we might need to factor out the contract field, although not necessarily.

Footnotes

1. Note that the logic introduced in Add --remove-old-proofs command #735 showcased above is not more inefficient than doing a single lookup per contract since, according to the Python docs, any is evaluated lazily. But it does result in a more verbose/redundant syntax. ↩