Included changes:
- minimal fixes to the packet layout to avoid the __packed attribute when not
needed
- new packet type called UNICAST_4ADDR: in this packet it is possible to find
both source and destination node (in the classic UNICAST header only the
destination field exists).
- a new feature: Distributed ARP Table (D.A.T.). It aims to reduce ARP lookups
latency by means of a simil-DHT approach.