This type is backed by the `zcash_address` implementaion of
unified full viewing keys. It is intended for serialization
and parsing of UFVKs, and provides conversion functions that
allow for construction to and from ZcashdUnifiedFullViewingKey
values.
After 4.5.2, all wallets will be populated with an emergency
recovery phrase, and all future addresses will be derived from
the associated seed. To prevent potential loss of funds, we
require that the user explicitly invoke the `walletconfirmbackup`
RPC method to verify that they have backed up this seed.