wasm: look into using the Fetch API

Emscripten has a Fetch API.

https://kripken.github.io/emscripten-site/docs/api_reference/fetch.html

Currently, we use XMLHttpRequest for network operations.

This is to research whether or not we can move to the Fetch API. Last time I looked into it, Fetch was buggy and accessing the number of downloaded bytes was not working correctly.

 Using the fetch API, whether emscripten's or javascript's, would allow to receive data in a stream, as opposed to all at once.