loose nodes are now automatically registrered when their settled
counterparts are. also rewrite a small amount of wrapper code and node
registration to work with this.
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.
rewrite some node registration code, removing the so far unused loose
nodes, and putting in place some data that outlines how they might be
implemented in the future.
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.
