refactor: make multi-threading more efficient.

[zicklag]

Current status. Working pretty well, but there are two issues:

• Somehow, I'm randomly leaving behind 1 sand pixel occasionally that floats in mid-air.
• There are still panics due to the the call to swappable() that tries to borrow neighboring cells, that are being mutated by another thread.

[zicklag]

OK, I pushed an update that makes all the borrows ahead of time.

• It will do a mutable borrow of the center chunk in a 9x9 grid.
• It will do immutable borrows of the surrounding 8 chunks.

Each thread will do this for it's active chunk and work at the same time.

I implemented Index and IndexMut for ChunkGroup, so you can use it almost like the vec you had earlier, and it will panic if you try to mutably access the surrounding non-mutable chunks.

Running the game, it will immediately panic because something is trying to mutate the sleeping chunks. I think maybe now ( whenever you get the chance ) would be a good time for you to check it out and see if you can figure out why it's writing to the immutable chunks.

Other than that, I think this design should work pretty well and be nice and fast.