We also add support for parsing Orchard full viewing keys from encoded
UFVKs (rather than treating them as unknown). `UnifiedSpendingKey` still
does not have Orchard support, so `UnifiedFullViewingKey`s will be
generated without Orchard components.
This is a breaking change to the database format. We don't have support
for migrations yet, so existing wallets won't work after this commit
until zcash/librustzcash#489 is done.