using the new falling node hooks provided by `bg_api`, add alternative
versions of existing stoneworking recipes, where instead of pummeling, a
stoney node must fall onto the input nodes with sufficient force.
note that since the only currently existing stoney node that can fall,
loose cobblestone, is unobtainable without cheats, so are these recipes.
make the wrapper `blockgame.register_node` make reasonable assumptions
that make node registration more convenient.
change other code to take advantage of that change.
also remove the `blockgame.reg_simple_node` utility since it's now
redundant.
pummeling definition tables now has lists of item id:s and groups, the
same way e.g. ABM:s do, that determine what items are involved in a
recipe.
update existing (in-use) pummel recipes to make use of this.
clean up things that were broken after renaming `bg_core` to
`bg_terrain`.
also clean up after global rename that added `bg_` prefix to every mod
name.
rename mods; add `bg_` prefix.
add hacky cleanup ABM (couldn't get LBM:s to work), that converts old
nodes into new ones according to this rename.
the cleanup doesn't do anything for items, though.
