Allow filters in the Packet slice / .getlayer() API

With a packet `pkt = IP() / IP(ttl=3) / IP()`, this will allow:

  - pkt.getlayer(IP, ttl=3)
  - pkt[IP, {"ttl": 3}]

This is particularly useful with Dot11Elt() layers, when you want a
specific value (the SSID or supported rates for example).