unseal method
Attempts to unseal the current slot revealing the private key for the
active slot. If isUsedUp is true this will fail. spendCode
must be a
6-digit numeric code
Implementation
Future<Slot> unseal(Transport transport, String spendCode) =>
Implementation.instance
.satscardUnseal(transport, spendCode, handle)
.then((value) => _sync(value));