Saves roughly 0.8s when running the tests, which took 16.6s and now take
15.8 (5%).
Removes the larger sample size, which replaced the closest selected
decoy with the real spend, per advice of Rucknium.
Changes the output index to a u8. While it may expand to a u16 at some
point, this can remain canonical using little endian serialization while
dropping the latter byte if it's 0 (or simply only using u16 when it's
actually possible).
