To do:
! Shane's output style.
! Match Shane's numbers

! All-at-once version?
  - make Nmin be just the print range.
  - also the print_walk range.
* rename walk_count, count_walks ... lib sth or other?

* memoize length-N array of counts for neat one-line reporting?
* Makefile(s) ... yamm the .mk's & then keep them.
* Make a README.txt file

* fscan_walk, sscan_walk.
* Print out full walks (in x0y0LRUD format) as a side effect, to
  argv-specified file.
* LRUD to xyxy converter exe, and vice versa.
* Include load-from-file option.  Worth doing?  If we want N=20, it's about
  the same time whether we've pre-computed N=10 or not -- is this true?