[tracking] browser WASM support

It'd be cool if `async-std` worked in the browser out of the box. Not everything can be supported, but we *can* support a smaller subset.

## Tasks
- [x] no longer fail compilation under the `--target wasm32-unknown-unknown` target
- [x] make `task::spawn` work (use [`wasm_bindgen_futures::spawn_local`](https://docs.rs/wasm-bindgen-futures/0.3.27/wasm_bindgen_futures/fn.spawn_local.html))
- [ ] expose `stream`
- [ ] expose `task::sleep`
- [ ] patch `async-attributes` to allow `async fn main` to work

## References
- https://docs.rs/wasm-bindgen-futures/0.3.27/wasm_bindgen_futures/
- https://github.com/rustasync/runtime/issues/7