Fix simple direction preferences. Grr.
Remove unnecessary direction + special-case fn.

I thought this was necessary because I had the prioritized directions in the
wrong order, so that after an :u I had to make it a special case to go :d
immediately. Putting things in the correct [:l :d :u :r] order makes this
problem go away.
Add AI priority-dir fn that panics sometimes.

Once the number of blanks exceeds a threshold, force a 3rd-priority dir
prioritization to line up offset-by-one matches and slam them together.
Reformat comment.
Add play-ai-dir-freqs fn to show movement direction frequencies.