Summary:
I couldn't find anything that consumes `--json` output, so I made this. It prints out benchmark results from two runs, comparing each benchmark to its corresponding baseline. This should help with benchmarking changes across revisions.

Along the way:
- Two small transparent `struct`s replaces the use of `tuple`s everywhere.
- New output JSON format (additive), formatted as array of arrays so order is preserved.
- New comparison binary in `folly/tools/benchmark_compare`

Sample output:

{P58307694}

Reviewed By: yfeldblum

Differential Revision: D5908017

fbshipit-source-id: d7411e22b459db16bd897f656e48ea4e896cb1bf