roconnor-blockstream

I've added two commits which I will squash afterwards.

squashed some commits.

> * Is it worth saying a short sentence about the motivation for introducing a second checksum? I've elaborated on why we are introducing a second checksum. > * To...

I guess another alternative is to have the first payload be 32 characters so that it decodes to exactly 20 bytes. I'm leaning towards the 30 character prefix because it...

> Why is it illegal to decode? `Any incomplete group at the end MUST be 4 bits or less`

> If we are going to do something other than "blocks of 93 characters, with the last one truncated" I'd rather try to make both blocks equal in size. BIP-32...

I think the potential money losing(?) proposition of incorrectly implemented decoders for rare long strings is pretty scary and this proposal does help mitigate that somewhat.

I get it. But I still somewhat prefer the tradeoff of `ms1` and then getting an extra index character.

Also I think numbering every input square with its ordinal position.

According to https://github.com/WebOfTrustInfo/rwot8-barcelona/blob/master/topics-and-advance-readings/social-key-recovery.md (referenced from SLIP-39) people may want multi-level SSS in order to facilitate social recovery subject to various restrictions.