…aming exceptions

…generic in debug output

The `unreachable!` previously acted as a safeguard in case the
translation of C's "no return" indicator would not work, ensuring the
code path is not followed any further -- but the linter didn't like the
(currently) unreachable code. Instead, we now implicitly assert that the
panic function does not return by not explicitly having a return in that
branch.

The `forget()` call was on the (Copy, hence a warning that is being
fixed) raw pointer instead of the underlying refcounted structure,
leading to a double free.

The lint is right in that dropping the reference does little, but it
ensures that the reference is not held longer than the lock. Using an
explicit scope achieves the same without any discussions about the drop.

The module can exist and be tested without all of GNRC