adds some benchmark for factorial

researched the factorial a little and found that iterative solutions should be faster.
Added some benchmarks to check how it is, and it really only matters for really big numbers.
And off course recursive throws stack exceptions at some point. 

Point is off course that llvm shines really brightly north of a few hundred.

PS: i put this on a brach so you can do the automatic merge and decide locally if you want it included in master. Don't know your policy here.