Mirage 1.0 support and Lwt-core

• The cohttp.lwt-core is now installed as an OS-independent Lwt library.
• Add support for Mirage 1.0, via cohttp.mirage-unix and cohttp.mirage-xen.
• Add a new Cohttp.Connection module to manage server's connections identifiers.
• Share the same configuration type for the different server implementations.
• Add Accept_types module to the Cohttp pack.