Recursive Fibonacci function

[sls1005]

You are using `cpuTime`, which gives you the time spent for running the current 
process. Since there could be two or more processes, a result of `cpuTime` 
might not be the real time and can be shorter. However, C's `time()` gives the 
UNIX time, which counts every second after 1970, therefore the difference 
between every two UNIX times is always in real time. To get the real time in 
Nim, use [epochTime](https://nim-lang.org/docs/times.html#epochTime) instead.