make this MPLd

add ht to Package-Requires

make aliases for dashes

make feathers that can't drop still toggle

drop feathers

make empty spaces blowupable

make only blowupable items blow up.

add boulders

TODONE that we can insert a diagonal row

TODONE error-catching in sand//with-new-board

make sand//with-new-board reset the board even when there's an error.

make things fall into smoke better

add TODO about with-new-board catching errors

make everything that changes state make a checkpoint.

let the ? key also print what's there.

fix bug where a balloon moving diagonally up didn't respect the things diagonally up

add tests for add-diagonal-angles

given we renamed sink to black-hole, change the tests too.

better explain interactive argument to add-diagonal-angles

add a way to add diagonal angles in a row