Added some functionality for marking a piece as a king. Added row/column function.

***NOTE: Code isn't 100% working.***

Lines 139 and 145: Commented out because it's complaining that I'm going from State to State&.

Line 210: I only added one is(Red,... call because there's an error. This needs to be called on an object. I was having issues getting that to work. For example, in legal_move, it seemed like piece was the most reasonable object to call this function on and it wasn't working. Additionally, you aren't calling is(Empty,... on an object, which concerns me.