Request only the Eio capabilities from env that are actually needed

[vog]

This implements the recommendation "Passing env" from the Eio documentation:

• https://github.com/ocaml-multicore/eio#passing-env

Among others, this makes Piaf easier to use in applications where only the actually required capabilities are passed around, avoiding the need to define dummy capabilities just to fulfill Piaf's interface.

To provide a specific example, it helps to reduce code like this:

let http_request ~sw ~clock ~net ... =
  let env = object
    method clock = clock
    method net = net
    method stdin = assert false
    method stdout = assert false
    method stderr = assert false
    method domain_mgr = assert false
    method process_mgr = assert false
    method mono_clock = assert false
    method fs = assert false
    method cwd = assert false
    method secure_random = assert false
    method debug = assert false
    method backend_id = assert false
  end in
  let client = Piaf.Client.create env ~sw ... in
  ...

to that:

let http_request ~sw ~clock ~net ... =
  let env = object method clock = clock method net = net end in
  let client = Piaf.Client.create env ~sw ... in
  ...

Some remarks about the implementation:

• The helper types required_env (client.ml) and clock (server.ml) simplify the type definition and casting required to put the Eio capabilities into a record.
• The sub function order_v4v6 has been moved out of Connection.resolve_host to make the latter polymorphic in its env parameter. Otherwise, calling Connection.resolve_host would be more cumbersome, as each call would require an explicit downcast to required_env (or a copy of that type in Connection).
• The definition of resolve_host could be simplified to resolve_host env ~config ~port hostname =, but I kept the current style to avoid messing around with the indentation, which would make the PR harder to read.