Replace <cassert> usage with a PANIC based assert

[pflanze]

The assert from #include <cassert> aborts on failures. Instead write one that uses our new PANIC macro, and include or use that instead.

[pflanze]

As per @Taepper's hint, C's (cassert) assert is eliminated if -DNDEBUG (https://en.cppreference.com/w/cpp/error/assert). Could emulate this.

Or could e.g. offer an ASSERT which checks the assertion even if NDEBUG is set, and an NASSERT which behaves like assert from C. Because most assertions could stay in release builds without the performance downside (Todo: overview over other project's approaches.)

[Taepper]

I see, I like this, fill me in on the details on Monday?

[fengelniederhammer]

Was done by #546 and #571