Use `ConstantTimeEq` for `UnifiedFullViewingKey` comparisons.

[nuttycom]

This should work but is quite ugly. I don't see any reason why keys shouldn't implement ConstantTimeEq.

Originally posted by @daira in https://github.com/zcash/librustzcash/pull/1213#discussion_r1509665137

This will require the implementation of ConstantTimeEq for sapling-crypto and orchard key types, which will be nice to have but is not immediately necessary.