Support for interfaces

[MOZGIII]

Sometimes you don't have a smart contract, but just a bare interface ABI you want to work with.

An example of the ABI spec:

[
    {
        "inputs": [
            {
                "internalType": "uint64",
                "name": "ticket",
                "type": "uint64"
            }
        ],
        "name": "check_payment",
        "outputs": [
            {
                "internalType": "bool",
                "name": "",
                "type": "bool"
            }
        ],
        "stateMutability": "view",
        "type": "function"
    }
]

This ABI spec requires the following declaration to function:

ethcontract::contract!("resources/abi.json", contract = CheckPayment);

Yet, the CheckPayment type is not generated.