packet lib: Simpler API to assemble a packet

For example:

(Current)
e = ethernet.ethernet(…)
i = ipv4.ipv4(…)
u = udp.udp(…)
pkt = packet.Packet()
pkt.add_protocols(e)
pkt.add_protocols(i)
pkt.add_protocols(u)

(New)
e = ethernet.ethernet(…)
i = ipv4.ipv4(…)
u = udp.udp(…)
pkt = e/i/u

Signed-off-by: Satoshi Kobayashi <satoshi-k@stratosphere.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>