Implement Mnemonic Codes w/ Shamir's Secret-Sharing

The master key is incredibly important and a combination of best practices enables a powerful backup strategy: 
1) use mnemonic codes
2) divide those codes into a multi-sig structure using Shamir's Secret-Sharing

[SLIP-0039](https://github.com/satoshilabs/slips/blob/master/slip-0039.md) provides a powerful example.