Node HTTP/2 was using the default h2 `Bytes` datatype when we can be
making using of `BufView` like we do in `Deno.serve`.
`fetch` and `Deno.serverHttp` can't make use of `BufView` because they
are using `reqwest` which is stuck on hyper 0.x at this time.
Adds `buffers` to the `Deno.jupyter.broadcast` API to send binary data
via comms. This affords the ability to send binary data via websockets
to the jupyter widget frontend.
