Handle multiple master public keys in a single transaction context

[spalladino]

We want to be able to get decryption keys or nullifier secrets for any address within the execution of a tx, not just msg.sender or the tx originator. Use case being having a public key shared among a group of people, and we want to read/modify that shared state in a tx, along with some state that belongs only to us.

Example:

// group is an address constructed from a shared public key

contract Lottery {
    function paywinner(group: Address) {
        require(msg.sender in [member1, member2...]);
        let prize = get_note_for(group);
        // consume and pay prize...
    }
}

[spalladino]

Fixed in #1110