new BIP: codex32

This introduces "codex32", a wallet seed/share format that uses the bech32 alphabet, a bech32-like checksum, and supports Shamir Secret Sharing, all possible without the use of electronic computers (although you can, and for some steps probably should, use computers if you want). Mailing list discussion: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-February/021469.html Website with more docs about hand computation (which isn't really covered by the BIP): https://secretcodex32.co

Specification