$ cargo run --quiet --release --bin fib_naive
          0.00000038s fib(10) = 55
          0.00001726s fib(20) = 6765
          0.00241714s fib(30) = 832040
          0.26354901s fib(40) = 102334155
         34.14185397s fib(50) = 12586269025
       4163.54114702s fib(60) = 1548008755920