Shrink framer

Currently, `framer-802154.c` creates a structure that describes the 802.15.4 headers. This structure is then passed to `frame802154.c`, which does the actual task of writing the 802.15.4 headers to the packetbuf. I suggest avoiding this intermediary step by directly writing the 802.15.4 headers within  `framer-802154.c`. This saves RAM and ROM. What I have not tested much are my modifications to `sicslowmac.c`, but otherwise it seems to work flawlessly and is also a bit faster.