Allow constants in interfaces

[axic]

(Moved this from #3411 which, now confusingly, refers to a different feature.)

Since we allow accessing constants through contract names (#1290) and enums in interfaces (#4087), it may make sense to reconsider this feature, at least for some healthy discussion.

I am still not sure if it is a good idea or not (it definitely can be useful from a users' perspective, but could be dangerous?)

The case I ran into is the following (ralexstokes/deposit-verifier#2):

interface DepositContract {
    uint constant PUBLIC_KEY_LENGTH = 48;
    uint constant SIGNATURE_LENGTH = 96;

    function deposit(
        bytes calldata publicKey,
        bytes32 withdrawalCredentials,
        bytes calldata signature,
        bytes32 depositDataRoot
    ) external payable;
}

In this example the interface uses basic types (bytes), but they have a limit enforced (the two constant above) so it would be useful from the caller to have require statements:

contract DepositProxy {
    function verifyAndDeposit(...) public {
        require(publicKey.length == DepositContract.PUBLIC_KEY_LENGTH, "incorrectly sized public key");
        require(signature.length == DepositContract.SIGNATURE_LENGTH, "incorrectly sized signature");

        // ...

        DepositContract(0x...).deposit(...);
    }
}

[axic]

Note, #8772 fixes the use case I would need this for. Not sure if there are other common use cases.

[axic]

This would be still a nice feature.

[github-actions]

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[github-actions]

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