In this commit, we introduce the concept of scopes for individual key
managers. Each scope will lock down a key manager to a particular
purpose and coin type within the BIP0043 hierarchy. Each scope will
also have a set address type schema. This schema will be consulted when
creating addresses for a particular scoped key manager.
Finally, we introduce 3 new default scopes:
* BIP 44
* BIP 84
* BIP 49++ (BIP49 but uses p2wkh for change addresses)
