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.

[balat]

Thank you Sam! This is very useful.
This adds a dependency to Tyxml that should probably be added in dune-project/opam
Could we avoid this dependency?

[shym]

I’ve inlined the code from TyXML to avoid the dependency to escape text and I used Ocsigen_lib.Url.encode ~plus:false to encode the file names as URLs.
This made me think about the question of encoding for the text: no charset is set on the reply and it’s not completely clear to me what would be the best choice there (especially since file names can contain any bytes but two on POSIX OSes, so not all of them make sense as text). Could/should I enable a standard mechanism to set the charset, for instance?