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.