Benchmark: timing 100000 iterations of factorial_i, factorial_i_extra, factorial_r, factorial_r_extra...
factorial_i:  5 wallclock secs ( 4.94 usr +  0.00 sys =  4.94 CPU) @ 20251.11/s (n=100000)
factorial_i_extra:  6 wallclock secs ( 5.67 usr +  0.00 sys =  5.67 CPU) @ 17633.57/s (n=100000)
factorial_r:  6 wallclock secs ( 6.44 usr +  0.01 sys =  6.45 CPU) @ 15496.67/s (n=100000)
factorial_r_extra: 13 wallclock secs (11.98 usr +  0.00 sys = 11.98 CPU) @ 8344.46/s (n=100000)