Hierarchy for Deterministic Multisig Wallets.

Sets a standard key layout for shared wallets that need several keys to spend, whatever the address type.

Earlier layouts for these multi-signature wallets were tied to one address type or kept a list of types. This BIP uses branch 87 for all of them and leaves the address type to the wallet's 'descriptor', a record of the keys and script used, which must also be backed up.