v2.1.0

CHANGES:

• support tcpip 4.1.0, which Ipv4.Fragments interface changed from LRU.M to LRU.F
• Mirage_nat_lru uses as well a LRU.F again
• breaking: Nat_packet.of_ipv4_frame / of_ethernet_frame output a pair of
  Fragments.t * (t option, error) result
• all in #37 by @hannesm