Recover EncryptedMasterSecrets from arbitrary sequences of Mnemonics

[pjkundert]

Recovering SLIP-39 EncryptedMasterSecrets from (possibly corrupted or otherwise attacked) sets of Mnemonics is the sole purpose of the SLIP-39 standard.

Verifying, grouping and vetting sets of mnemonics requires at least partial decoding of the mnemonic to extract identifier, extendable flag, group counts, thresholds, etc., so that only compatible mnemonics are considered. This is difficult to do "externally" to the SLIP-39 implementation.

Therefore, a robust API to recover one or more SLIP-39-encoded encrypted master secrets from a pool of collected mnemonics is not just useful, but critical to the proper operation of a SLIP-39 based recovery system.

Thus: I propose shamir_mnemonic.group_ems_mnemonics, which takes a sequence of Mnemonics (as either str or Share), and produces a sequence of EncryptedMasterSecrets and a dict of group indices and the list of Mnemonics used to recover the secret. It does so in a manner resilient to various corruptions or attacks, ignoring invalid, unrelated/incompatible or redundant Mnemonics.

Fixes #44