codex32: Checksummed SSSS-aware BIP32 seeds.

Would let people back up a wallet's master secret as a checked code, optionally split into shares, even by hand with pen and paper.

Fewer backups are safer from theft, but more are safer from loss. codex32 can split the secret into up to 31 shares, with a chosen number (2 to 9) needed to restore it; fewer shares reveal nothing. A checksum catches mistakes and can be verified by hand.