Task: benchmark convex/convex collisions.

[paulmartel]

Benchmarks are needed to motivate and inform and monitor the possible changes described in issue #14 and possibly other future rework.

[w0rm]

I've taken a slightly different approach to measuring performance. Elm benchmark is not suitable for macro-benchmarks, so what I did was set up a scene with 64 cubes and used that to check FPS and use the JavaScript profiler to identify performance bottlenecks. I was able to improve it quite a bit.