17810: drivers/slipdev: implement sleep states r=benpicco a=benpicco
18348: sys/net/gnrc/pktbuf_static: make use of alignas() r=maribu a=maribu
### Contribution description
Since we are now using C11, we can make use of `alignas()` provided by `<stdalign.h>` to make the alignment code easier to read.
### Testing procedure
I didn't expect this to change binaries, but is safes 4 bytes. `elf_diff` shows that the compiler (at least GCC 11.3.0) was not able to detect that `gnrc_pktbuf_static_buf` was just an alias for `_pktbuf_buf`. That makes sense since it would be hard without LTO to rule out external writes to `gnrc_pktbuf_static_buf`, unless one would have added a `const` (to the pointer, not to the data the pointer points to).
The [output of `elf_diff`](https://mari-bu.de/pr_18348_gnrc_pktbuf_static_elf_diff.html) looks otherwise quite unscary.
Also:
```
$ make BOARD=nucleo-f767zi -C tests/unittests/ tests-pktbuf flash test
make: Entering directory '/home/maribu/Repos/software/RIOT/tests/unittests'
Building application "tests_unittests" for "nucleo-f767zi" with MCU "stm32".
[...]
Welcome to pyterm!
Type '/exit' to exit.
READY
s
START
.............................................
OK (45 tests)
make: Leaving directory '/home/maribu/Repos/software/RIOT/tests/unittests'
```
### Issues/PRs references
None
19120: CI: seperate check-labels and check-commits workflows r=maribu a=kaspar030
Co-authored-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Co-authored-by: Kaspar Schleiser <kaspar@schleiser.de>
`tsrb` changed to that type, so there is no reason for us, to keep the
internally used buffer as a `char` (especially since the rest of the
driver already uses `uint8_t`)
This simplifies and solidifies the reversal of SLIP's byte-stuffing
(aka byte-unstuffing ;-)) by
1. Using `tsrb` instead of `ringbuffer`: there are two actors. The ISR
and the device's event handler.
2. Moving the byte-unstuffing from the UART RX-handler (i.e. the ISR)
to the device's receive function (potentially not the ISR)
3. Removing the `pktfifo` member. The current number of bytes in the
ringbuffer is returned for `recv(data = NULL, len = 0)`. If that is
more than the packet contains (due to the byte stuffing it most
likely will be) the packet is reallocated in GNRC anyway.
