non-brute-force pulling down algorithm
non-brute-force pulling down algorithm

instead, every time a column N was pulled down, sort columns to the left
and right and pull them down as well (recursively).

this may still result in weirdness in some configuration, but it at
least won't get stuck in an infinite loop.

[finishes #98752724]

zachgersh/atc,zachgersh/atc,pivotal-devin-fallak/atc,utako/atc,zachgersh/atc,utako/atc,utako/atc,zachgersh/atc,savaki/atc,simonjjones/atc,savaki/atc,simonjjones/atc,simonjjones/atc,pivotal-devin-fallak/atc,savaki/atc,pivotal-devin-fallak/atc