sys : net : Packet buffer

~~Based on #1101. Tests will follow~~

This PR adds a priority-based packet buffer datatype for the network stack.
Because in `packetbuf_remove` it the pointer could not be valid anymore after returning, while in `packetbuf_get` (which is expected to call somewhat often) the overhead of always copying the data into the pointer would be too much. Henceforce, it just returns the pointer (via call-by-reference).