More concurrency libraries

[Alizter]

Fiber is the concurrency library behind Dune https://github.com/ocaml-dune/fiber

Compared to lwt or async, fiber implements what is known as structured concurrency. This allows for processes to always have a parent process (or a pool) to (error) report to. This also allows for a meaningful collection of values allowing for parallelism on top of concurrency.

Fiber is very minimal and doesn't even have a scheduler. In Dune we have our own scheduler, but you can also use something like lev-fiber https://github.com/rgrinberg/lev.

[bluddy]

Feel free to open a PR.