refactor(furvm): implement thing constructor

furvm/include/furvm/thing.hpp

@@ -51,7 +51,7 @@ private:
     thing_t                  m_type;
     std::shared_ptr<context> m_context;
 
-    void* m_data;
+    void* m_data = nullptr;
 };
 
 } // namespace furvm

furvm/src/thing.cpp

@@ -0,0 +1,10 @@
+#include "furvm/thing.hpp"
+
+namespace furvm {
+
+thing::thing(rzecz, thing_handle id, thing_t type, const std::shared_ptr<context>& context)
+  : m_id(id), m_type(type), m_context(context) {}
+
+thing::~thing() {}
+
+} // namespace furvm