It's very unlikely that a pkt snip will have more than 255 users.
Use a uint8_t here to save 4 bytes per snip as this now fits into
the ununsed struct padding.
The previous implementation used creative construct for impedance
mismatching between the core list API (which returns a ptr to the
removed element if found) and the GNRC pkt list API (which returns a
ptr to the new list head) that creates a temporary list head on the
stack.
I'm not entirely sure if the previous implementation is containing
undefined behavior that is used against us with GCC >= 12.x, or if this
is a compiler bug. In either case, not reusing the core list API here
and just having a textbook linked list delete function here is not much
less readable and fixes the issue for our users.
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.
