Added cWorld:ForEachBlockEntityInChunk() and cWorld:DoWithBlockEntityAt() functions.

Also exported them to the Lua API.
Debuggers: Test harness for cWorld:ForEachBlockEntityInChunk().

The fill command will fill all empty slots in block entities with containment with gold nuggets, one per slot.
APIDump: Documented cWorld:ForEachBlockEntityInChunk() and cWorld:DoWithBlockEntityAt().