[aggregations] filter more efficiently

[onlyjsmith]

We're iterating every response for every filter. How can we reduce this?

• [ ] Smarter indexing: Segment Trees?
• [x] Combine multiple predicate functions into a single function - i.e. multiple filter elements into one filter. We'd then only iterate the array of responses once. (done in #154)
• [ ] Freezing or using immutable arrays (it's immutable data anyway)
• [x] for instead of filter? really surely we're getting some benefits from some transpilation step? But note that this isn't based on any measurements - the loop might be a tiny part of the overall time cost. (done in #154)

[onlyjsmith]

Looks to me like Array.filter is exactly the same in the transpiled output. (Searched for filter_responses in app.js on deployed stage).

I think this would be a great place to start looking at performance, with a very small surface area of code to touch, and limited chance of side-effects.