fix tailrec version of recursion

[melrief]

Tail Recursion should be as faster as a while-loop, sometimes even faster because of internal compiler optimizations. If it's not the case then the developer has done something wrong. In the case of your code, the tail recursion version of fac the "slowness" of the implementation is caused by how you implemented fac, which is different from the while example, and has nothing to do with tail recursion. I've attached to this email a screenshot of the score of my tail recursion implementation (ending with 2) to show that tail recursion is on pair with the while loop. I have only tested Long because I don't have time for the other three tests but I'm fairly confident the result will be similar.