Crashes when running GC stress test

- Executing the [scratch-files/gc-stress-test.ch](https://github.com/KCreate/charly-vm/blob/rewrite/scratch-files/gc-stress-test.ch) file sometimes results in a crash.
- Crash is not deterministic
- Three different types of crashes observed
  - `unexpected worker state` in `worker.cpp`
  - Failed atomic cas somewhere in `worker.cpp`
  - General bus error (segfault?)