Get rid of the hack to retrieve private keys in Cooked.Wallet.walletStakingSK

[florentc]

To retrieve the private staking key of a wallet, we need to unwrap it from type Pl.MockPrivateKey. There is no way (explicitly exported constructor or field accessor) to do so. Therefore we use a hack in which we define some isomorphic datatype newtype HACK = HACK Cardano.XPrv and unsafeCoerce the Pl.MockPrivateKey to HACK.
We should try to make a PR to expose access to the info in their API.
Also we could try to rely on Data.Tuple.Solo (from base 4.16 which we don't have in our current index-state) to avoid defining HACK.