3.0.0

CHANGES:

• remove Lwt.t monad, lwt dependency (#47 @hannesm)
• remove ppx_deriving dependency (#47 @hannesm)
• revise mutable state: use "mutable foo : x" instead of "foo : x ref" (#47 @hannesm)
• add Mirage_nat.is_port_free (#47 @hannesm)
• revise Mirage_nat.add to take a port_generator (unit -> int option) (#47 @hannesm)