feat(furvm): add collect function to context

furvm/src/context.cpp

@@ -1,7 +1,16 @@
 #include "furvm/context.hpp"
 
+#include "furvm/thing.hpp" // IWYU pragma: keep
+
 namespace furvm {
 
 context::context() {}
 
+void context::collect() {
+    for (auto& ref : m_things) {
+        if (ref->reference_count() != 0) continue;
+        ref = nullptr;
+    }
+}
+
 } // namespace furvm