Maze generation

Basic maze generation, new data structure, algorithm dont works well so
far, cell stack not used, drawing debug gizmos in editor. For now use
DFS script only now. Commented unused code.
Maze generation

Still doesnt work
Maze generation 2

Beginig from scratch, this time track "exits" instead of "walls",
separated IGrid interface, Debug drawing script, use enum bitflags for
marking exits in cell, todo: algorithm implementation