Finish basic implementation of garbage collection.

include/gc.h

@@ -33,6 +33,10 @@ void libab_gc_list_init(libab_gc_list* list);
  * @param data the data to pass to the visitor.
  */
 void libab_gc_visit_children(struct libab_ref_s* ref, libab_visitor_function_ptr visitor, void* data);
+/**
+ * Applies the given visitor function to this reference.
+ */
+void libab_gc_visit(struct libab_ref_s* ref, libab_visitor_function_ptr visitor, void* data);
 /**
  * Adds the given reference to the given garbage collection list,
  * and specifies a function used to reach its children.

include/refcount_internal.h

@@ -0,0 +1,8 @@
+#ifndef LIBABACUS_REFCOUNT_INTERNAL_H
+#define LIBABACUS_REFCOUNT_INTERNAL_H
+
+#include "refcount.h"
+
+void libab_ref_count_changed(libab_ref_count* count);
+
+#endif