v4.0.0

CHANGES:

• Adapt to mirage-protocols 4.0.0, mirage-net 3.0.0, mirage-time 2.0.0,
  mirage-clock 3.0.0, mirage-stack 2.0.0 interface changes (#420 @hannesm)
• Revise Static_ipv4.connect signature (for more safety):
  val connect : ip:(Ipaddr.V4.Prefix.t * Ipaddr.V4.t) -> ?gateway:Ipaddr.V4.t ->
  ?fragment_cache_size:int -> E.t -> A.t -> t Lwt.t
  it used to be:
  val connect : ?ip:Ipaddr.V4.t -> ?network:Ipaddr.V4.Prefix.t ->
  ?gateway:Ipaddr.V4.t option -> C.t -> E.t -> A.t -> t Lwt.t
  The clock C.t is gone (due to mirage-clock 3.0.0), ~ip and ~network are
  now required and passed as pair ~ip. The optional argument ?gateway is
  of type Ipaddr.V4.t. The new optional labeled argument ~fragment_cache_size
  specifies the byte size of the IPv4 fragment cache (#420 @hannesm)