Move Eio.Stdenv.t to Eio_unix.Stdenv.base

[talex5]

The old type wasn't directly useful for anything, since nothing provides exactly this set of resources, and nothing needs to consume exactly this set either. However, it is useful in Eio_unix as the set of resources that a backend needs to provide to be compatible with Eio_main.run.

The new type is almost the same for now, except that the standard streams include file descriptors. Other Unix-specific features can be added later, such as support for passing file descriptors over sockets or to child processes.

I called the type base because we might want to add a Stdenv.linux_or_unix subtype later for basically everything that isn't Windows. Not sure what the name should be, though.

Note that, due to the way row-polymorphism works in OCaml, we can't e.g. have Net.connect return an Eio_unix.socket subtype of Eio.Net.socket. But we can add a new function (e.g. connect_unix) that returns a richer socket type.

[c-cube]

if I want to ask for "classic" permissions that a non-Eio program would have, what should I use now? I started something with 0.9 and exactly one stdenv:Eio.Stdenv.t -> … signature and now I'm a bit lost. I'm not interested in tying it to a particular backend either.

[talex5]

If your program requires a "classic" OS then you can just use Eio_unix.Stdenv.t instead. It will work with any of the classic backends (eio_linux, eio_posix, eio_windows). It won't work with unikernels, but it wouldn't have done with the old Stdenv.t either.

But probably it would be better to list the things your program actually needs. See: https://github.com/ocaml-multicore/eio#passing-env