Added a specialized class for drawing operations (lines, bezier curves,
cuboids, etc..., more will be added in the future!)

The player_transaction and world_transaction classes have been removed
in favor of the new edit_stage class!

Drawing 2D-3D primitives is pretty simple:

    dense_edit_stage es (wr); // wr = pointer to world
    draw_ops draw (es);

    // draw a line from {0,0,0} to {5,7,9} using red wool blocks
    es.draw_line ({0, 0, 0}, {5, 7, 9}, {BT_WOOL, 14});

    // commit changes to the world
    es.commit ();

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Added /cuboid, /line, /bezier and /aid.