This adds two new commands to wrench:

perf - This runs all the benchmarks in the benchmarks manifest, and records
the timing of various profile counters into a JSON file. This is
very similar to how the reftest code works.

compare_perf - This allows comparing two files output from the perf command above.

There are two new workflows enabled by this change:

(1) Working locally:
 - Run `perf` to get a baseline benchmark.
 - Develop new feature or optimization.
 - Run `perf` to get new results, and use `compare_perf` to compare to baseline.

(2) Automated performance measurement:
 - This is not available yet, but the idea is to upload the results
   of the perf command to a web server, which graphs the results
   over time.

New benchmarks can be added to the benchmarks/ directory, in a similar
way to adding reftests.