made some breaking changes to minesweeper
made some breaking changes to minesweeper

- rows and columns are now letter-based, rather than number based.
  this allows me to use emojis for the rows and columns on the edges.

- I redid the deferring of the mine-placing yet again. Basically I fell back
  to what I did originally, and just stored an internal attribute for the mine
  count. This allows for some cute troll games like the famous 10 x 10 with 90 mines.

Ikusaba-san/Chiaki-Nanami