PR for WebXR in Rust wasm-bindgen

Hi all,

I've opened a pull request to put WebXR bindings in Rust via wasm-bindgen -
https://github.com/rustwasm/wasm-bindgen/pull/2000.

There were a couple of issues creating the bindings which I've documented
with TODOs in the WebIDL (still unresolved). Most of them are to do with
FrozenArray, but I also had to comment out most of the attributes
on XRWebGLLayer.

The PR wont get merged until the spec gets beyond draft, but it's available
as a branch if anyone wants to play around with it.

Thanks for the work you guys have been doing on WebXR, I've been following
with interest for a while now.

Thanks,

Kev