Eio backend for JavaScript environments

[vouillon]

This provides:

• eio_js_backend, a simple backend;
• js_of_ocaml-eio, a counterpart to js_of_ocaml-lwt;
• eio_brr, Eio support for Brr (Eio variants of Brr functions that returns directly instead of returning a future).

This has been heavily inspired by #405. Compared to this PR, there is a clear separation between the scheduler and the JavaScript bindings. Also, it is possible to use Eio from JavaScript callbacks (e.g. from event handlers) as was attempted in #534.

The scheduler is quite different from other Eio backends. It provides a start function that starts executing asynchronously a function in a context where Eio effects can be performed instead of a blocking run function. Indeed, one cannot block waiting for events in JavaScript but one needs to return to the JavaScript event loop. The start function can be called in JavaScript callbacks to use Eio.

eio_jsoo is not complete yet.

[talex5]

Thanks - this looks great (I only read the eio_js_backend.ml file; it looks sensible to me).

For others who want to try the example:

1. You need to install opam install js_of_ocaml-ppx manually (this is why CI is failing).
2. You need to dune build @default to get the resource files. dune build alone isn't enough.
3. Opening the index.html in a browser doesn't work. The Firefox console doesn't show anything by default, but if you switch to "Multiprocess (slower)" then it shows "Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at file:///home/user/work/eio/_build/default/lib_js_of_ocaml_eio/examples/boulderdash/maps.txt. (Reason: CORS request not http)". To fix that, I ran docker run --rm -it -p8080:80 -v (pwd)/_build/default/lib_js_of_ocaml_eio/examples/boulderdash/:/usr/share/nginx/html:ro nginx and then opened http://localhost:8080/.
4. Then I was able to play the example game, and it worked.

[talex5]

The code has now moved to https://github.com/ocaml-multicore/eio_js, so closing this PR.