Improve show delta computation via swap-vals!

[brunchboy]

In the Zulip chat, Ineffable introduced me to swap-vals!, a core library function I had previously not noticed. I should take advantage of this in the code that needs to compare before-and-after cue states when a status or beat packet is received in the show code, instead of manually keeping around the old values in a corner of the global myself. It would be a more efficient, and—clearly—idiomatic approach.