Adopted use of map-indexed.

Also, ditched unused, commented-out helper function.
Added docstrings for neighbor-y functions.
Added function that returns sets of pos's crucial for win.
Added function to test for a win for player X.

Also, corrected lame positional from winning-positionals (had
copy-and-paste-ridden duplicates).
Cleaned up organization of tests.
Added predicate that tests if board is fully marked.