First pass at a sudoku solver: Puzzle Import

Imports a puzzle from a string and displays the restricted puzzle.

solver/puzzle.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <optional>
+#include <string>
+
+#include "cell.h"
+
+class Puzzle {
+ public:
+  static Puzzle FromString(std::string puzzle);
+
+  std::string CurrentState();
+  std::string PencilMarkState();
+
+  void AssignSquare(uint8_t id, uint8_t value);
+
+ private:
+  Puzzle();
+  std::array<Cell, 81> cells_;
+};