feat: Add fuzzing for cipher/xor

[abeltay]

Following up from #480, adding fuzzing test to cipher/xor function.

I ran go test -fuzz=Fuzz -fuzztime 30s cipher/xor/*.go and got the output below:

fuzz: elapsed: 0s, gathering baseline coverage: 0/8 completed
fuzz: elapsed: 0s, gathering baseline coverage: 8/8 completed, now fuzzing with 10 workers
fuzz: elapsed: 3s, execs: 1541453 (513640/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 6s, execs: 3078539 (512364/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 9s, execs: 4648655 (523545/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 12s, execs: 6233252 (528124/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 15s, execs: 7810894 (525865/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 18s, execs: 9394482 (527848/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 21s, execs: 10964800 (523534/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 24s, execs: 12536121 (523682/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 27s, execs: 14119233 (527786/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 30s, execs: 15706281 (529005/sec), new interesting: 0 (total: 8)
fuzz: elapsed: 30s, execs: 15706281 (0/sec), new interesting: 0 (total: 8)
PASS
ok      command-line-arguments  30.481s