Add support for directory listings in Staticmod

[shym]

The goal of this PR is to implement the "FIXME: staticmod dirs not implemented". The implementation might be too naïve to be included, at least as is. In particular:

• the generation must load the full directory (so that the entries can be sorted),
• consequently the generation should probably not be enabled when there are huge directories (which are usually a bad idea anyhow),
• the listings are unstyled.

The idea was to make it so that

let () =
  let dir = Sys.getcwd () in
  Ocsigen_server.start
    ~ports:[ (`All, 8080) ]
    ~command_pipe:"/tmp/ocsigenserver-cmd" ~datadir:"/tmp" ~logdir:"/tmp" ~debugmode:true
    [
      Ocsigen_server.host
        [ Extendconfiguration.listdirs true; Staticmod.run ~dir () ];
    ]

is a possible ocsigenserver version of python3 -m http.server.