Bunnymark example

[kvark]

This is technically ready. Interestingly, using wgpu logo made it run slower for me. I checked, and we appear to be texture unit bound on GPU, so the logo is probably just bigger than the other picture.

[kvark]

Let's add it with an idea that:

1. we'll potentially move the examples out into a separate repo. I think this has a benefit of allowing users to copy stuff out of the examples, easier.
2. we'd extend this example with more features, such as TypedArena use.

bors r=grovesNL

[bors[bot]]

Build succeeded:

• build (macos-10.15)
• build (ubuntu-18.04)
• build (windows-2019)
• docs
• wasm