* initial chasms mod
* tweak default chasm settings
* prevent chasms from breaching oil and magma seas, make veinstone actually do something
* overgenerate caverns to eliminate floating stalactites
* make veinstone punchable instead of right-clickable
* ensure dfcaverns get carved before chasms
this has an unfortunate tradeoff. Chasms will no longer have floating giant columns in them, but will also no longer have smaller stalactites and stalagmites. Also will carve chasms through lake water. Not sure if this is ideal.
* add rare big webs to the chasms, to give them a unique feature
* reverse the dependencies for df_caverns and chasms, let chasms go first.
* fix web generator
* add webs to level 3 tunnels, fix sunless sea chasms
* fix up tunnel webs
* make webs snappy
* make webs slightly more prevalent
* add chasms to the guide
* final touch-ups before merging
* allow anchoring against unloaded blocks
