2 warmup iterations should be a default, or something

[dabrahams]

Swift builds up lots of metadata and witness table caches at runtime, and when timing things that go quickly, that can be really significant. At least if the iterations are fast enough, you should throw away the first one to get those caches initialized, and maybe throw out another, to get the fast paths into the i-cache. I found adding warmup iterations could drastically change the relative measurement of some benchmarks.