total_supply should be optional or a shielded field

[AndrewWestberg]

On public blockchains, it's desirable for maximum transparency. On a privacy blockchain like midnight, total_supply of a token can disclose activity to others that might be an undesirable side-effect. For example, on bitcoin, everyone is constantly watching whale activity because it might indicate a large selloff and the market usually reacts to it in anticipation.

On midnight, we should optionally be able to keep total_supply as a private field so that mint/burn amounts are not disclosed.

[AndrewWestberg]

For example, to keep total_supply private, we could change the definition from

export ledger total_supply: Map<Bytes<32>, Uint<128>>;

to

export ledger total_supply: Bytes<32>;

Then, assuming this token is only mintable by a central authority (such as a stablecoin issuer), the minting should be updated to shield the total_supply behind a hash only the authority is capable of producing.

// Only the central authority will know the current_total_supply
export circuit mint(current_total_supply: Uint<128>, ...) {
  assert( ** that the localSecretKey() has the authority to mint **)
  assert(total_supply == totalSupplyHash(localSecretKey(), current_total_supply as Field as Bytes<32>))
  ...
  // do the mint
  ...
  // update the total_supply privately
  total_supply = disclose(totalSupplyHash(localSecretKey(), (current_total_supply + amount_minted) as Field as Bytes<32>))
}

circuit totalSupplyHash(sk: Bytes<32>, instance: Bytes<32>): Bytes<32> {
    return persistentHash<Vector<3, Bytes<32>>>(
        [pad(32, "my_privacy_token:pk:"), instance, sk]
    );
}