Cardano support with Shamir

[ZdenekSL]

Child of #269
Fix Cardano support with Shamir seed.

[tsusanka]

@refi93 we are introducing a new form of recovery system, called Shamir backup, defined in SLIP-0039 (a somewhat replacement for BIP-39). I'm trying to figure out how to support Cardano on top of this, in particular here.

You are calling bip32.from_mnemonic_cardano, which converts the BIP-39 mnemonic to entropy and then calls hdnode_from_seed_cardano. I'm a bit confused why not call bip39.mnemonic_to_seed() before and insert the seed into hdnode_from_seed_cardano. Is there any difference? Or do we need to call
mnemonic_to_entropy, which is needed for Cardano?

I know it's quite some time ago already, so let me know if you don't know. Thanks!

[andrewkozlik]

I created SLIP-0023 to solve this issue, see https://github.com/satoshilabs/slips/blob/andrewkozlik/slip-0023/slip-0023.md. In https://forum.cardano.org/t/cardano-with-slip-0039/25443 I asked people to make comments on the new specification in the pull request satoshilabs/slips#712.

I opened a pull request #359, but it needs careful testing.

[andrewkozlik]

I tested #359 on the device to make sure that it gives the same addresses as the SLIP-0023 test vectors. So it should be fine, but testers should also try some Cardano transactions on a Trezor using SLIP-0039 to ensure that everything is in order.