Implement and use fair_adjacent_iterator.

Also correcting the spore explosion directional bias introduced in
0.16-a0-2679-gdd6be42.
Simplify some set logic.
Don't pretend a few functions can fail.
Don't reduce crawlie merge chance when full aboms are adjacent.
Move statue and wisp form rPetrify into form data.
Make player shadows insubstantial (AreBrandon)

At least for rPetrify purposes.