feat: introduce reference instruction

furvm/include/furvm/instruction.hpp

@@ -40,6 +40,13 @@ enum class instruction_t : byte {
      */
     Clone,
 
+    /**
+     * @brief Pushes a new reference onto the stack.
+     *
+     * Pops the top thing from the stack and pushes its reference.
+     */
+    Reference,
+
     /**
      * @brief Adds two things together on the stack.
      */

furvm/include/furvm/thing.hpp

@@ -368,7 +368,7 @@ public:
     }
 
     thing reference() const {
-        thing res              = { std::make_shared<type>({ m_type }), m_allocator };
+        thing res              = { std::make_shared<class type>(m_type), m_allocator };
         res.get<reference_t>() = m_data;
         return std::move(res);
     }