rust examples for calculating PI.
test bench_multiple_thread ... bench: 22,729,990 ns/iter (+/- 39,968,474)
test bench_single_thread ... bench: 77,571 ns/iter (+/- 59,599)
test bench_with_may ... bench: 4,431,440 ns/iter (+/- 3,119,797)
test bench_with_rayon ... bench: 54,298 ns/iter (+/- 20,816)