Further improve Cohttp_eio.Client ergonomics

[bikallem]

While working on documentation for cohttp-eio and #957, it dawned on me that the Cohttp_eio.Client ergonomics could be further improved. Such that users of Cohttp_eio.Client don't even need to use Eio.Net.with_tcp_connect for most use cases. This PR aims to do just that. Now we do,

open Cohttp_eio

let () =
  Eio_main.run @@ fun env ->
  let res = Client.get env ~host:"www.example.org" "/" in
  print_string @@ Client.read_fixed res

rather than a bit verbose below:

open Cohttp_eio

let () =
  let host, port = ("www.example.org", 80) in
  Eio_main.run @@ fun env ->
  Eio.Net.with_tcp_connect ~host ~service:(string_of_int port) env#net
    (fun conn ->
      let host = (host, Some port) in
      let res = Client.get ~conn host "/" in
      print_string @@ Client.read_fixed res)

Note, however, we still retain the ability to pass in the flow/connection parameter like before. It is however now an optional parameter rather than a required one.

[mseri]

why did we decide to not use uri here?
A uri will contain all the information at once

[bikallem]

Not really sure now but if I remember right being able to use the flow/connection was one I think. It wasn't obvious to me then but now I think it was the correct design. Additionally, I believe we can't use unix domain sockets in url without inventing our own syntax since http uri spec. doesn't specify how we can use unix domain socket scheme in the uri.

[mseri]

That makes sense, the main reason I am asking is because right now it feels weird to separate the host from the path (mostly because any other library will take that all at once) but this is surely an improvement over the previous version

[mseri]

Should we try to be smart and add / as the first character if not already present?

[bikallem]

Should we try to be smart and add / as the first character if not already present?

I think that is a good idea but not sure atm. However, I would like to address that in future PR please if we do go down that route.

[bikallem]

I have created a new issue so that we can discuss it further. #959