Add initial classes and a couple of tests for Board

src/board.cr

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+
+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

src/parser.cr

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+module Sudoku
+  # Parse a sudoku board from a string. Return a board.
+  class Parser
+    def self.parse(board_str)
+    end
+  end
+end

src/position.cr

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+module Sudoku
+
+  struct Position
+    
+  end
+
+end

src/solver.cr

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+module Sudoku
+  class Solver
+    def self.solve(board)
+    end
+  end
+end

src/sudoku.cr

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+# TODO: Write documentation for `Sudoku`
+
+require "./*"
+
+module Sudoku
+  VERSION = "0.1.0"
+end