Reduce use of postmessage

* Wasm has the atomic wait and atomic notify functions, it should be possible for the workers to wait until woken up to reduce the use of `postmessage` calls.
* Could it also be made possible to have `wasm-bindgen` as an optional dependency?
We are working on a web [game](https://github.com/squirrel-labs/ratatosk/) and consider using anmethyst or rather SPECS as our ECS.
* Is the run_notify used in any way by rayon, or could this be omitted for simplicity?

I'll try to fork this when I get around to implementing our thread pool