Added mark-at? predicate.
Renamed position->string to mark->string.
Added docstrings for functions.
Changed positions to be [row, col] pair.

Also, changed place-mark-at to ignore attempts to
clobber marks at already-marked positions.
Flipped place-mark-at to map with index.

Learning that Clojure's map function behaves like some
other language's zip when map is given more than one coll.
Created functions for positions and open-positions.