Wasm compatibility

[feefladder]

as discussed in #72, clients in wasm need some special handling using channels. The main question here is where that special handling should be done. I thought something like

async fn maybe_make_send<O, F: Future<Output = O>>(f: impl FnOnce() -> F) -> O
{
    #[cfg(feature = "wasm-compat")]
    {
        let (tx, rx) = oneshot::channel();
        spawn_local(async move {
            let result = f().await;
            tx.send(result); // can error, at which point we do nothing
        });
        rx.await.unwrap(); // error means rx dropped, could retry?
    }

    #[cfg(not(feature = "wasm-compat"))]
    f().await
}

which could wrap something like

impl AsyncFileReader for ReqwestReader {
  fn get_bytes(&self, range: Range<u64>) -> BoxFuture<'_, AsyncTiffResult<Bytes>> {
    let client = self.client.clone()
    let href = self.href.clone()
    maybe_make_send(async move {
      Ok(
        client
        .get(href)
        .header("Range", format!("bytes={}-{}",range.start,range.end+1))
        .send().await?
        .bytes().await?)
    }).boxed()
  }
}

Otherwise, this would be its own crate and implementations from here are copied over, which is also fine

[kylebarron]

I don't think at this point there's anything needed to put into async-tiff to maintain wasm compatibility. I think it's fine for small helpers like this to be in external crates.

[feefladder]

Ok, great!