StructureMapFragment takes far too much memory.

[IntegratedQuantum]

It ends up using ~1 MB of memory per instance. This is way too much.
I need to find a way to reduce the number of structures it considers in each chunk, and maybe find a better memory allocation strategy than the arena.

Required changes:

• free the empty memory at the end of the Arena
• Construct all the arrays on the stack first, and later transfer them to the heap arena, to reduce fragmentation inside the Arena,
• Reduce the number of structures per chunk by using less conservative structure sizes (this would require generating the CaveMap to figure out where the structure starts)

[IntegratedQuantum]

Now it seems pretty good to me. Further optimization doesn't seem necessary for now, but I'll leave this open for the future, in case it becomes a problem again.