includes just enough code to try to auto-route them as they're placed.
the "root" node is now the horizontal one - if you have diagonals and
they don't place right, they should after you dig them and place again.
when placing, code will try to determine if the bannister should be
left- or right-sided. Defaults to left-sided; hold shift while placing
to force right-sided.
create it by placing two twins of the same color side by side, left first
also moved the bed extension functions into handlers/expansion.lua,
renamed them, and made a few other minor tweaks to that file accordingly.
