This is awesome, thank you!

One issue we ran into when I was looking into wasm a while ago was that arrow buffers had to be copied into wasm memory. Is this able to read `contents` directly?

Not sure. https://github.com/rustwasm/wasm-bindgen/issues/1659#issuecomment-510925917 seems to indicate that we need a copy but maybe reference types fix that. I am not using reference types yet (because wasm-pack doesn't have it yet https://github.com/rustwasm/wasm-pack/pull/937).