Minor fix: Sudoku (redesign grid constraint)

[joshuaguerin]

Current grid constraint is ugly and not terribly elegant. I think a solution is to be had that involves div or mod that greatly reduces the complexity of the final 2 constraints.

[joshuaguerin]

I was correct. Div appears to correctly identify the grid location of a variable (or will once corrected arithmetically), removing the need for a reference to explicit blocks. Updated in 39dcfc6.

I like where this is a lot better. I'm not convinced that I cannot make this read slightly better, so I'm going to leave the issue open for future consideration with fresh eyes. I think I've at least reached "not embarrassing" territory.