ls.go: Implement Fast User/Group ID Conversion

The lookup function in os/user is incredibly slow and there is no official method of obtaining group names for group IDs. Therefore, I have written my own functions to convert uids and gids into usernames and groupnames with great success. Instead of taking 250-300 milliseconds and lacking the capability to convert group names, I am now accomplishing perfect user and group id conversion in 47 milliseconds. The C version of ls takes 70 milliseconds, so we have a win.