Disambiguate `Benchmark#cycles` and the `onCycle` event from `Benchmark#stats.samples.length`

[jdalton]

The number of times onCycle is called doesn't match up with Benchmark#cycles because onCycle is also called per sample run as well.

[andrewdeandrade]

How about onBenchmark for each benchmark and onSample for each sample, and then Benchmark#cycles can now be a private counter.