Swap packet_size and packet_type field locations

[rmarx]

see quicwg/qlog#40

packet_type is now in PacketHeader
packet_size is now renamed and no longer in PacketHeader, but in raw.length on the individual events.

first pass probably: swap them to draft-01 places if they are already reversed (draft-02)
second pass: properly update the tools to look at the new places and then swap everything pre-draft02 to draft02

cc @marten-seemann