Instead of memmoving a packet's data around this patch instead moves
around the packet pointer itself.

The only performance testing I did was "snabb lwaftr bench", where it
appears to be neutral-to-slight-win.  I am hoping that the Hydra/R
scientists can help me out on this one.

This change to the packet structure does have a knock-on effect, in
two significant ways.  One is that the shiftleft, shiftright, and
prepend functions now return a new packet pointer and silently corrupt
the old one (because probably they shifted around the data but still
had to update the new "length" pointer).  That's somewhat OK though.

The problem comes in the datagram library, which sometimes wants to
work on a user-supplied packet.  The sequence was:

  local dgram = datagram:new(pkt)
  dgram:push(....)
  dgram:free()
  ...
  foo(pkt)

This pattern is no longer valid :(  The reason is that the datagram
methods that mutate a packet often do so by packet.prepend().  The
datagram library updates its internal packet pointer, but the external
one that was initially passed in is no longer valid.  So much of this
patch is visiting (hopefully) all of the uses/users and updating them.