Add setsockopt() to eio sockets

[bikallem]

This PR implements setsockopt() function to Eio.Net.socket.

Fixes #322

[haesbaert]

I can't reason much about why changing the type declaration to classes, it seems to be a change for the sake of change ? Or what am I missing. If so, it would make more sense to put those on a different commit to make reviewer easier.

Since you're providing setsockopt, should we provide getsockopt as well ?

[bikallem]

Since you're providing setsockopt, should we provide getsockopt as well ?

Indeed, I was thinking along the same lines but wanted to get this PR reviewed and merged first. I have opened issue #376 for this.

[talex5]

It's not actually necessary to create a separate Mock.Net.stream_socket type (which is an API change). We could just have Flow.t implement the socket methods too. It already works as both a source and a sink, for example.

We could just add the new method to the implementations in eio_linux, etc, rather than using inheritance, to shrink the PR. Though this seems fine to me either way.

[bikallem]

It's not actually necessary to create a separate Mock.Net.stream_socket type (which is an API change). We could just have Flow.t implement the socket methods too. It already works as both a source and a sink, for example.

We could just add the new method to the implementations in eio_linux, etc, rather than using inheritance, to shrink the PR. Though this seems fine to me either way.

Indeed, this is an api change; however, I think this api addition should be mostly backwards compatible since I am not sure if there is too much (any?) code in the wild which inherits from Eio.Net.socket class.

Additionally and indeed, I agonized a bit on where to put the new function - Eio.Flow.two_wya, Eio.Flow.source or Eio.Flow.sink. However, this line made the decision for me (https://github.com/ocaml-multicore/eio/blob/main/lib_eio/flow.mli#L1). IIUC, flow types are a general/common abstraction over files and network sockets. As such it didn't seem coherent/cogent to me to put it in one of the flow types, i.e. how would we implement setsockopt on file flow type? Eio.Net.socket seemed to fit the requirement as it is a super class for both udp and tcp socket and is an abstraction for sockets.

[bikallem]

Ah, just realized you meant only the mock flow type. I don't really have a strong preference for either in the mock case. I think I mostly followed the abstraction is the Eio.Net.socket as I didn't want to muddy the semantics even if it is for mock use case.

[bikallem]

@talex5 @haesbaert please approve/merge this PR is there are no more objections.

[bikallem]

closing this for now.