Don't require Unix in order to use mock sockets

[quernd]

Thanks for always being at the forefront and adapting to breaking changes in Eio's API!

After the Eio objects to variants migration, we weren't able to use an Eio_mock.Net network for tests anymore because commit 0c672bf changed the socket type in the Client module from Eio.Flow.two_way to Eio_unix.Net.stream_socket_ty Eio.Net.stream_socket.

This is in contrast to the Server module where it's still _ Eio.Net.stream_socket.

As far as I can tell, there's no necessity for this restriction, so this PR changes it back and exposes socket as Eio.Flow.two_way again. ([ `Generic ] Eio.Net.stream_socket_ty Eio.Net.stream_socket would also be possible and maybe more appropriate; I went with two_way because that's what it was before.)

However I don't know if there is any code that relies on it being Unix instead of the more generic type. I wasn't able to find any, but if this was a conscious choice maybe we can find a different way to allow for mock tests and still expose the socket (I refrained from adding a type parameter to Client.t for the network type but that would be one way, I guess).

If you think this is change feasible, there's a one-line change in h2, httpaf etc. needed as well so that they can also use mock networks again, along the lines of this commit. I can open those PRs but they only make sense in conjunction with this one.

[anmonteiro]

I believe my thinking at the time was that two_way sockets didn't provide close, but I could be misremembering. I'll test this soon and let you know.

[quernd]

I believe my thinking at the time was that two_way sockets didn't provide close, but I could be misremembering. I'll test this soon and let you know.

Well, you're right: two_way flows don't provide close. If it's important that this be exposed then maybe socket can be coerced to type [ `Generic ] Eio.Net.stream_socket_ty Eio.Net.stream_socket or something similar? It would provide close but not be tied to Unix, and it would also work fine with our mock tests.

[wokalski]

[ `Generic ] Eio.Net.stream_socket_ty Eio.Net.stream_socket or [ `Close | Eio.Flow.two_way_ty ] Eio.Flow.two_way or carrying the 'net generic around will all work. depending on the design goals of the val socket function carrying 'net around or coercion is probably preferred.

[wokalski]

@anmonteiro rebased on top of the latest changes.

[anmonteiro]

appreciate your patience here, give me a bit more time.

[anmonteiro]

Thanks!