int on_dig( int delta )

This function is passed a signed coordinate, giving the direction of
digging. This is appropriate for obstacles that you dig through, such as the
rock in mines. If the player specified only that they wish to dig the
object, the delta will be 0 (not SMAKE_C(0,0,0), but actually 0). If the dig
succeeds, the on_dig function should return 1; if not, then return 0. If
delta is 0, then the player is already standing next to the object to be
dug, and you do not need to check. Otherwise, you need to check.