While it is necessary in the worst case to perform `m * n` decryptions,
where `m` is the number of outputs being decrypted and `n` is the number
of IVKs, it is possible to stop performing trial decryptions when the
first successful decryption is performed. Also, it's inconvenient and
unnecessary to return the full cartesian product of these results, as
only one IVK will decrypt a given output. This commit modifies batch
trial decryption to stop on the first successful decryption, and instead
of returning the cartesian product of results we return the index of the
input IVK along with the output it decrypted. Note that this means that
trial decryption is not constant-time with respect to the number and/or
order of IVKs.
Extracted from: 5873950648
Jack Grigg
Daira Hopwood
Kris Nuttycombe