Recovery tables for the random share(s)

[roconnor-blockstream]

We can add tables to recover the value of the random share in case someone loses that share. With both the secret and random shares recovered, one can then in turn recreate any other missing share, or even add new shares (as long as the share threshold isn't increased).

In principle any other share can directly be recovered from any pair of shares. While creating a recovery table for every possible share might not be reasonable, we could make it very easy for advanced users to programmatically such tables for themselves if they want.