refactor(furvm): use generational indexes for thing

Refs: #10

furvm/include/furvm/context.hpp

@@ -5,6 +5,7 @@
 #include "furvm/fwd.hpp"
 #include "furvm/module.hpp"
 
+#include <queue>
 #include <type_traits>
 #include <vector>
 
@@ -21,12 +22,12 @@ public:
     /**
      * @brief Move constructor.
      */
-    context(context&&) = default;
+    context(context&&) noexcept = default;
 
     /**
      * @brief Move constructor.
      */
-    context& operator=(context&&) = default;
+    context& operator=(context&&) noexcept = default;
 
     context(const context&)            = delete;
     context& operator=(const context&) = delete;
@@ -97,8 +98,9 @@ private:
     std::vector<thing_p>    m_things;
     std::vector<executor_p> m_executors;
 
-    std::vector<void*> m_deadThings;
-    furlang::arena     m_thingArena;
+    std::queue<thing_handle> m_deadThings;
+    std::vector<void*>       m_deadThingData;
+    furlang::arena           m_thingArena;
 };
 
 } // namespace furvm

furvm/include/furvm/thing.hpp

@@ -50,7 +50,7 @@ public:
     const char* what() const noexcept override { return "bad thing access"; }
 };
 
-class thing {
+class thing final {
     friend class executor;
 private:
     /**
@@ -61,6 +61,8 @@ private:
     struct rzecz {
         explicit rzecz() = default;
     };
+public:
+    static constexpr thing_handle GENERATION_SIZE = 12; /**< Bit size of generation part in thing_handle. */
 public:
     /**
      * @brief Private constructor.