Block Corruption

Lots of minor changes, and some major ones as well. Some of these changes are listed below:

Corrupted zones (after becoming deactivated when a player moves too far away) can now become re-activated if a player comes nearby. Corruption points spawn naturally now. Corruption points now spread towards a player more often. Less CPU usage due to corruption point exclusion. Blocks placed by a player are now immune to corruption (as are certain important blocks such as end portal frames). Added block validation in many areas to ensure sanity. And more!™

Blocks placed by a player being immune to corruption is achieved by spawning a third (new) kind of invisible entity. The entity lasts for 51 seconds (1024 ticks).