lib/protocol: faster Luhn algorithm and better testing (#6475)

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)

lib/protocol/deviceid.go

@@ -165,7 +165,7 @@ func luhnify(s string) (string, error) {
 	for i := 0; i < 4; i++ {
 		p := s[i*13 : (i+1)*13]
 		copy(res[i*(13+1):], p)
-		l, err := luhnBase32.generate(p)
+		l, err := luhn32(p)
 		if err != nil {
 			return "", err
 		}
@@ -183,7 +183,7 @@ func unluhnify(s string) (string, error) {
 	for i := 0; i < 4; i++ {
 		p := s[i*(13+1) : (i+1)*(13+1)-1]
 		copy(res[i*13:], p)
-		l, err := luhnBase32.generate(p)
+		l, err := luhn32(p)
 		if err != nil {
 			return "", err
 		}