Removed most delegations, #clue_list became clues.
Traversal of the grid is now in Grid::Traverser.

Traverser can move word-wise and cell-wise.
Changed highlight for more subtlety and clarity.
Added a possibility to create from a (x, y) pair.
Cluelist is now Grid::Cluelist
The cursor keys now navigate the grid.
Grid#word_from_pos became word_num_from_pos.