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).
Honestly, the borrowed keys are frustrating, and this probably reduces
performance while no longer offering an order when iterating. That said,
they enable full u16 indexing and should mildly improve the API.
Cleans the Proof of Knowledge handling present in key gen.
