Gets the decryption speed to the same level as the
encryption speed.
internal/speed$ benchstat old.txt new.txt
name old time/op new time/op delta
StupidXchacha-4 732MB/s ± 0% 740MB/s ± 0% ~ (p=1.000 n=1+1)
StupidXchachaDecrypt-4 602MB/s ± 0% 741MB/s ± 0% ~ (p=1.000 n=1+1)