Use Enum.map for getting diagonal cells

Basically, a more confusing way of doing:

    left_diagonal = [
      Enum.at(grid.cells, 0),
      Enum.at(grid.cells, 4),
      Enum.at(grid.cells, 8)
    ]

    right_diagonal = [
      Enum.at(grid.cells, 2),
      Enum.at(grid.cells, 4),
      Enum.at(grid.cells, 6)
    ]