Stress-test performance suite

Closes #120

Would love a review @tyoshino and anyone else. If you want to read the minimal amount necessary to give a helpful critique, I'd suggest [reference-implementation/bench/README.md](https://github.com/whatwg/streams/blob/perf-test-suite/reference-implementation/bench/README.md) and [reference-implementation/bench/params.js](https://github.com/whatwg/streams/blob/perf-test-suite/reference-implementation/bench/params.js).

Excited to use this thing. Hope I did it right.
`Interval` may be more descriptive than `Rate`?