Add Leibniz formula for Pi benchmark

[leewei05]

It took about 2 seconds to calculate the current approximation of Pi. If we add another 0 to the number of loops, it would take about 20 seconds to add another digit, so I just stop here.

1000000:

$ time bril2json < leibniz.bril | brili
3.14159365359077425
________________________________________________________
Executed in    2.28 secs    fish           external
   usr time    2.15 secs    0.52 millis    2.15 secs
   sys time    0.09 secs    1.62 millis    0.09 secs

10000000:

$ time bril2json < leibniz.bril | brili
3.14159275358980139
________________________________________________________
Executed in   20.98 secs    fish           external
   usr time   20.43 secs    0.44 millis   20.43 secs
   sys time    0.27 secs    1.43 millis    0.27 secs

[sampsyo]

This is SUPER cool! Thank you!!