Use `Readable.from` instead of `StreamxAsNodeDuplex`

We may be able to use `Readable.from()` instead of [`StreamxAsNodeDuplex`](https://github.com/digidem/secret-stream-http/blob/b9a9206b289bdcde60b0cc1466291f8ff0d58b8a/src/streamx-as-node-duplex.js) to convert the streamx instance from `@hyperswarm/secret-stream` into something that Node's http server will be happy with. This would be better, because it reduces maintenance of this library.