Add queen attack

[ageron]

I think this exercise is a good opportunity to use an opaque type.

Square := { row : U8, column : U8 } is an opaque type that represents a chessboard square, and it is guaranteed to be valid (i.e., row and column are both between 0 and 7). This illustrates the fact that opaque types can enforce constraints.

I also wanted to show that the implementation details could be hidden from the user, so I didn't want to simply provide row and column functions. Instead, the exercise asks the student to write rank and file functions. This also has the additional benefit of reducing the risk of name clashes in the code (in my first implementation, I had a row and column function, and it was a bit painful because several variables had to be named r or c to avoid clashes).

[isaacvando]

Looks great! This is a really nice way to use opaque types 😃