Implement pass by pointer

[claude-betz]

We need to be able to pass in the address of a variable (pointer) as an argument to a function and modify the variable.

void
modify_num(int *i)
{
    *i = 2;
}

int
main()
{
    int num;
    num = 1;
    modify_num(&num);

    /* num = 2 */
}