Created freelist.lua: general purpose freelist data structure.
packet.h: Extended and updated.

Added "fuel" field to packets. This is like the IPv4 TTL and will be
used to drop packets that are looping within Snabb Switch, i.e.
prevent switching loops within a snabbswitch from becoming
catastrophic.

Renamed packet_header to packet_info and packet.buffers to packet.iovecs.
Wrote packet.lua and rewrote buffer.lua. Not tested yet.

Packets and buffers are both allocated from freelists. Packets are
reference counted and buffers are not. Freeing a packet automatically
frees all of its buffers, so the only way to ref() a buffer is to
ref() the packet that owns it.