Up until now `utlist.h` is used to manipulate the packet list within a
GNRC pkt snip. Since I always wanted to see if there would be an
advantage when using `core`'s `list.h`, I provide this
`gnrc_pktsnip_t`-specific API to manipulate the packet list. It has the
additional advantage of type safety and that future steps could
encapsulate those operations.