Exotherm - Memory conservation

The virtual server that I'll hopefully be able to run Exotherm on has only 1 GB of RAM. I've noticed that Exotherm uses a sizable fraction of that, so if I want my server to be able to continue doing more important things, I'll need to be careful about memory usage. I noticed that the "think" module keeps board objects for all positions in the tree even after they've been evaluated, stopping the GC from collecting them. So I adjusted it to set them to None when no longer needed, allowing the GC to make progress even in the middle of a think if necessary.