This feature reserves a big virtual memory area during startup (0x40000000 bytes, so 1 gigabyte. this should maybe be more, since virtual memory is essentially free) and gives it to mimalloc for use to allocate nurseries in.

This lets us check very cheaply if a pointer points into a nursery, or somewhere else. Most importantly, checking if an object is in generation two now becomes a simple bounds check on the pointer (plus check if the feature is currently in use, since the virtual memory reservation at the start can fail) instead of having to follow the pointer which is not guaranteed to hit the processor cache (especially in a minor gc collection run).