Generation might need contextual information

[rom1504]

For example it might need to be able to generate 2x2 chunks to place a tree between 2 chunks.

In order to do that I propose to switch from a generation function being (chunkX,chunkZ) -> chunk to (world,chunkX,chunkZ) -> promise

That way the generation function would just add whatever it needs (and query whatever it needs) before saying "I'm done".

It would be a "make sure that chunk is ready" function instead of being a "give me that chunk" function

[rom1504]

Apparently the "place a tree between 2 chunks" is done in a population second phase. Maybe it would make sense to do that in a second time us too, I don't know. It would be a good idea only if performance requires it, right now it doesn't.

For now, the generation will probably stay as it is.