Support for codex32/BIP93 seed import

[apoelstra]

Hi all,

I would like to support importing seeds in the BIP93 format, which consists of either

• A bech32-encoded BIP32 seed, of length 128-512 bits, with a 13-character checksum (15 characters for 300 bits or more)
• A collection of bech32-encoded BIP32 seed shares, which can be reconstructed into a seed using SSSS.

I am happy to do 100% of the implementation and integration work. We will require a new dependency, rust-codex32, though I am working on the existing dependency rust-bech32 to make it generic over checksum types, which may make rust-codex32 small enough that we can just inline the essential funcitonality.

See https://secretcodex32.com/index.html for more information.

Would people be interested/accepting of this?

[notmandatory]

This looks like a valuable addition to me.

If it adds a new dependency I think it'd be best to add a new optional feature like what we did for bip39, but if it becomes part of rust-bech32 it shouldn't need a feature flag. Will this only require a little glue code like impl<Ctx: ScriptContext> DerivableKey<Ctx> for Codex32String ?

[apoelstra]

@notmandatory yeah, not much more than that. Though it won't be DerivableKey but probably something else because codex32 seeds are BIP32 seeds rather than keys. (The same applies to bip39 seed words; I'll just copy whatever API exists for that.)

Sure, I'm happy to have a feature gate as long as we need an extra dep.

[apoelstra]

FYI this is blocked on resolving BlockstreamResearch/codex32#58 which determines the exact import rules. It won't be quite as simple as I imagined because I'd like to write an API that answers all the user-facing questions about e.g. what to do about erroneous inputs, repeated shares, etc.