Add a collatz conjecture benchmark

[ljedrz]

Doubles as a straightforward example.

Note: the benchmark values are not random; they are the values with the largest stopping time in their range; citing Wikipedia:

The starting value having the largest total stopping time while being less than 10 is 9, which has 19 steps, (...) less than 10^5 is 77031, which has 350 steps, (...) less than 10^10 is 9780657630, which has 1132 steps,

[ekzhang]

Great! This is a cool example. Here's the output on my computer, it's about linear as we'd expect.

Thanks for the contribution.