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.
move plank registration and recipe out of tree mod into newly created
woodworking mod.
also update plank recipe to require cobblestone instead of stone to be
held while pummeling.
