Decide and document thread-safety in cross-platform API

[talex5]

Currently, we only say that the synchronisation primitives (promises, etc) can be safely shared between domains. However, we need to allow at least some other things (e.g. the file-system and network) to be shared as otherwise there's no way to use them from other domains. Probably all of Stdenv should be shareable.

For Unix, this may require some fairly heavy-weight reference counting to ensure that we never close a file descriptor while it's being used by another thread (which might include it being queued up in another domain's io_uring queue, for example). Unix is quick to reuse FD numbers once closed, which can easily lead to corrupting an unrelated file.

Also, we should document whether you can do e.g. overlapping writes on a flow. For example, whether this always writes "AB" to the flow f (see e.g. mirleft/ocaml-tls#458):

Fiber.both
  (fun () -> Flow.write f "A")
  (fun () -> Flow.write f "B")

[talex5]

OS resources can be safely shared now with the eio_linux and eio_posix backends. Once eio_luv is deleted, we can document that this is supported.