zcash_client_backend: Support SLIP-39 seed phrases.

[nuttycom]

Trezor wallets support the SLIP 39 standard for k-of-n seed phrase backup. We should make it possible for wallets based on zcash_client_backend to restore based upon SLIP 39 seed phrase shares.

https://content.trezor.io/slip39
https://github.com/satoshilabs/slips/blob/master/slip-0039.md

[ec2]

Would be cool to punt this up the stack and make zcash_client_backend only be aware of Zcash keys! This will make it much easier for people to implement different types of key stores. I think I've discussed this with @AArnott on Discord before.