Runner: merge stats objects and logPerfEvent

[mhofman]

Originally the runner took the approach of logging JSON perf events to a file, representing various events, and capturing CPU and memory usage at regular intervals.

In order to generate stat summaries and more structured data, #36 is introducing stats objects, which are saved wholesale at the end of a run.

We should merge the approaches so that:

• perf events are still generated, but from the stats structure instead of explicit calls
• perf events could be used to "stream" noteworthy events, raw block/cycle data, as well as intermediate summaries (e.g. per stage)
• CPU and memory usage would be integrated with other stats (see Include CPU and memory usage stats summary #42)
• it may no longer be necessary to dump a large "stats" event at the end containing all raw and summarized data

Since summaries as a single object is useful for jq processing, maybe there should be a way to regenerate the summarized stats object easily from the raw event stream.