Add initial classes and a couple of tests for Board

src/board.cr

@@ -0,0 +1,29 @@
+
+module Sudoku
+
+  class Board
+
+    alias Value = Nil | Int8
+
+    enum State
+      Valid
+      Invalid
+    end
+
+    getter grid
+
+    def initialize(@width : Int8, @height : Int8)
+      @grid = Array(Value).new(@width * @height, nil)
+    end
+
+    def put(value : Value, x, y)
+      @grid[y * @width + x] = value
+    end
+
+    def get(x, y)
+      @grid[y * @width + x]
+    end
+      
+  end
+
+end