The previous implementation was very generic; its tests didn't cover the
actual alphabet for device IDs.
Benchmark results on amd64:
name old time/op new time/op delta
Luhnify-8 1.00µs ± 1% 0.28µs ± 4% -72.38% (p=0.000 n=9+10)
Unluhnify-8 992ns ± 2% 274ns ± 1% -72.39% (p=0.000 n=10+9)